People always say Ronaldo going to last longer than Messi, but I'd think Messi would last just as long, if not longer.
You can see how consistent he was over past 10-15 years, the way he took good care of himself over the years was very notable.
The way he plays his football - covering so little distance on the pitch yet always delivers, would means any stamina decline wouldn't be a big issue for him.
He will definitely loss more pace and explosiveness as he ages though, and his no. of running/dribbling past players will decline too because of his declining pace, but he will still be very good at playmaking, ball control and finishing. I reckon he can still play at highest level when he is approaching his mid and late 30's.
But can he switch to midfield and play near as good as Iniesta? Not a chance!
Look, its one thing playing a "part-time" free role in midfield involving in buildup, playmaking and creating and all that at his will, its another thing to actually taking up all the responsiblities required in midfield, taking up right positions, chasing up and down, helping out teammates in the attack and defend, circulating possession, winning back possession, not losing your position and shapes of your team, keep moving around and sharing the defensive responsibility too. Messi, doesn't have enough of these traits to make him successful in full time midfield role. Its just like playing Rooney in midfield, or Alan Smith in midfield, or even worst. Don't get me wrong, he is definitely GOAT as attacker, or no.10, or playmaking wing forward, but he is not a superhuman type of footballer who can play every position equally well. He never was.
For example, Cantona looks more like a proper midfielder than Messi ever was (more aggression, more influential, lot tougher, great vision and passing, always drop back in midfield creating stuffs too, and very good at flying tackle in kung-fu style!), but I doubt very much he would be success in midfield too.