Their problem is the way the team is built, and it includes Mbappé. Obviously Mbappé would work with a team filled with top players at every position, but having him on the pitch with Vini decreases his influence and weakens their balance. It was a totally unnecessary addition and they'd been (Vini included) in a far better position had their money been invested differently. They're just not compatible.
Every GREAT player it's compatible in terms of style and atributes, yet not every player it's compatible in the sense of getting along with another player.
In PSG times, people talked too much of Ney, Killian and Messi not defending, yet it's not such a huge issue, or just the only one.
Killian and Ney constantly clashing with their egos was way worse, that kills chemistry, that makes players go for the individual play and glory and fight for silly stuff the whole time.
Mbappe got really late that Messi was there to assist him like no one ever before and by the time he got it, the team had already collapsed (Messi wrongly never wanted to be THE GUY and that also was a mistake).
From the usual bloopers from the defense, to Verratti never really coming back from his latest injury, Ney injured once again and Vitinha looking like a mal functioning clone in comparison with current one...while also not having precisly the best coaches out there and never replacing players like Di Maria and cia properly with mature new comers with similar traits. To reduce that period to Ney, Killian or Messi not defending, it was just silly, easy food for Social Media.
For instance that Barca of Suarez, Ney and Messi, it wasn't extraordinary the pressing they did (bar Lucho being himself), it was a team playing as such providing a proper basement to let them be and create upfront. Their chemistry made them almost take advantage of every play they've done.
YET, by the time cockiness was too much, specially after winning the 2015 CL, the team as a whole was more lazy, some players in the mid got older and the cracks started to be visible.
For instance in Real the relantionship between kid Vini and Benzema and cia wasn't precisly great when Vini was still maturing, yet they finally got their act together and they created a betetr chemistry between them. Now they have to do the same.
And at the end of the day, the main issue for Real, more than their forwards being more or less defensive, it's their MIDFIELD.
Like City, they let their mid get old in the "clever" department, they got a lot of muscle, stamina, talent, youth, yet Modric got ancient, Kross retired and like City, they didn't do the correct moves.
When after this the team faced quite a numbre of injuries (like City too) all that previous suddenly succees goes to the toilet. Everything that stays put, that doesn't move, in this world gets eaten alive.