^^^ Cristiano Ronaldo cannot be replaced by Aguero. It would change the dynamics of Real Madrid. Real Madrid have no structure and depend a lot on individual actioin rather than a systematic process.
There have been many games where Madrid could not get the ball into Cristiano's feet when he is running in behind because they don't have a clear passing pattern. Madrid can play terrible football with no control of the game and then simply cross the ball and Cristiano Ronaldo would score a header.
Now, you can sit here and say heading isn't a great skill but for the way real Madrid play, it is almost a high chance of goal creation. It's not as simple as replacing player X with player Y. What are the team team dynamics? What is the teams structure? Who are the leaders? What are the dynamics with the coach? Can the incoming player face the pressure of Madrid? Will the teamates give the incoming player enough of the ball to make a difference (Bale in his first 2 seasons)?
There is so much more involved in a game than analysing an individuals supposed skill set when adapting to a team. Noone would have predicted (save for me and anyone with an understanding of the position game) that Gabriel Jesus and eve Raheem Sterling would move Kun Aguero to the bench but according to this sub and the English media, Kun Aguero is a 'great' striker. Which is it? Gabriel Jesus is a better footballer? He is better for the team? Guardiola doesn't know what he is doing? Again, it is not a simple answer.
As for Messi, I have said this before on another forum, he is a double edged sword. You cannot use positional play with him and Suarez in the same team except you play 3 at the back, because Messi always moves into the centre from the half space and Suarez is good in the centre and is awful as a wide player.
What about his work rate? If Barcelona offered Manchester City Messi for a reasonable price now I am 100% sure that Guardiola would reject it and rightfully so. Messi's wages would hinder their squad building and his lack of pressing would rub off on the team. Cruijff has always said your team mirrors your best players. Messi's wage has put Barcelona at a disadvantage as well. So you can't use positional play, and this affects the way youth players will be integrated into the first team as they have all been schooled in that way from the ages of 8 - 19. He lacks no workrate and so in big games, you can see that Barcelona defend with 8 players as Suarez has stopped pressing as it is a waste of time because Messi doesn't cover the closest passing option so Suarez just ends up being lethargic.
Also, Messi forces the team to play through him. This is a similar issue that Cristiano had originally with Real Madrid where he was taking almost 7-9 shots per game and was more involved with ball carrying and I mentioned before he was a tactical liabilty. He was never a winger but essentially a second striker who sometimes occupied the wings so Marcelo was always isolated and so when people said he could not defend I used to always say, nobody would look great at defending if you had to defend 50m by yourself and are constantly outnumbered numerically. Now that he has a lesser role, he is far better for the team even though his ball carrying ability is almost relatively non-existent. he finishes off moves now that the team creates and plays his role, allwoing others who are superior in other areas to allow him to be closer to goal to do what he does best which is what football should be about.
In the 09/10 season where they lost to Inter I actually believe that not all of it was down to Zlatan Ibrahimovic as some people tried to blame it solely on him. Messi wanted to play Centrally and even sent Pep a text about it saying if he didn't play centrally he would be on the bench. So what does Pep do, start both Messi and Zlatan, two players who need and force teams to play through them in central aras and it was relatively easy for Inter Milan to defend. Messi didn't want to play on the right where he wouldn't have as much influence and be a cog rather than the whole wheel.
Messi was becoming all encompassing even in Pep Guardiola's final season and that was one of the contributing factors that contributed to them not being as good the season before. He worked less off the ball, wanted the ball to feet, didn't create space and open up the pitch for teammates with his running. He is more powerful than Barcelona and it shows, especially with the last contract negotiations. Ronaldo simply doesn't have that power at Real Madrid. His workrate got worse and worse and there are even some rumours that Messi was a contributing factor to Pep Guardiola leaving. Can you imagine Pep doing what he did to Aguero to Messi? As usual, Johan Cruijff was right...
Messi needs to have more and more of the ball and so he dominates the team, when he is stopped, there is noone that can pick up the batton. It is the same issue Zlatan Ibrahimovic has had in his career. Messi is having a ZLatan effect on the team. This is why they have been a failure in 4/5 Champions League campaigns where teams can just pack the centre, but can dominate LaLIga like PSG dominated Ligue1 under Zlatan.
Watch Argentina. It doesn't help when the manager comes out and says the team is based around Messi. What does that do for his teammates mentality when they have the ball. Messi for Argentina is static and only wants the ball to feet and his teammtes always are under pressure to pass to him. If he constantly moved and opened up passing lanes for his team, I bet Argentina would look a lot better than they do now. All the players run so hard except him. I point to the Copa America in 2016 where Argentina routed Chile in the group stage but struggled against them in the final. Why is that? Watch the game. They pass the ball to Messi who is static and is then surrounded by 6 players and then players run off the ball but he can't get the ball to them, Messi is then forced to rely on his dribbling from deep against 2 blocks of players, something that he couldn't even do regulary at the peak of his dribbling. It happened over and over again. Whereas in the first game, the team ran for each other and opened up passing lanes for each other so it was easier to get the ball into the box. They will be in trouble without Messi for sure now because al their training and tactics is geared to giving the ball to him.
Cristiano for Portugal in Euro 2016 was not outstanding but he perfromed his role. The team don't have to constantly pass to him like they did before. If Ronaldo was at the same physical level he was at in his prime, I am certain Portugal won't have won the Euros because he would have tried to be the protagonist. He is much wiser and so waits for the right moment to intervene. is shot selection has improved relatiely as well as he was taking the most shots per season in Europe every season since Opta started recording data for the top 5 leagues. This is a main factor in Madrid doing a lot better now than what they did in his 'prime' in the Champions League as Madrid have done much better since Roanldo has been physically declining.
Messi is the superior footballer but it takes a certain ingredients for him to operate to full capacity and certain tactics and he has to dominate the given team tactics. Ronaldo is less involved but easier to get to be part of a functioning team. Essentially, it is easier to get the best out of Cristiano at present and he is more adaptable but if things are right and specialised for Messi then he takes the team to another level.
At this moment in time, if I were to pick one of them in my team I would go for Cristiano.
I wonder why neither has won a single Ballon D'or whilst Ronaldo is on his way to a 6th.