There's going to be no need at all for Messi to move back into a position deeper than #10 any time in the next 3-5yrs unless injury befalls him or his fitness completely falls off a cliff.
Messi, when everything is stripped away, is a #10, and even in a declined state, he has physical attributes far beyond some of the greatest to ever play in that role. For example, Messi at even half the physical fitness he has now would still be a faster and more mobile player than a Riquelme, Valeron, Valderrama who were all notoriously slow play-makers with sublime passing ability and technical acumen. Unlike Valderrama and Riquelme, Messi does not have obscene upper-body strength, but what he does have in its stead to compensate, or probably even better, is one of the lowest centres of gravity and a level of core balance few in the history of the sport come close to matching. This will enable him to pass, give-and-go, and as nonchalantly as any of the aforementioned, endlessly link play whilst riding tackles and slipping by opposing players who are forced to over commit on their challenges.
Of course, Messi is likely never going to be that slow, as a worse version of himself, he'll still be a more explosive athlete than countless great #10's were at their absolute peaks. Players like Totti, Bochini, and Rivera could only dream of being so able to leave a man on his backside due to a sudden and complete shift in momentum and bodyweight as a declining Messi.
I think once people get over comparing Messi to Messi, there will be a realisation that well into his 30's, Messi will have attributes that will still be par with some of the greatest #10's there has been. The likes of Baggio, pre-injury Del Piero, Dalglish and even Platini are thereabouts what Messi will be for a good few years yet.
It's only the superhuman freaks, who had both the technical prowess and the physical gifts that he is branching away from now, the likes of Cruyff, Maradona, Himself, Ronaldinho, Kubala, Albert etc. etc. who were challenging enough to even catch or contest with physically that are becoming out of sight as templates.
The point of this post is to give some perspective about Messi's declining state. Even now, he has physical gifts that comfortably slot him into the category that matches the peak output some of the best #10's had at their very best. It stands to reason, then, that if these players could have the storied careers they did, Messi's new chapter, although different from the freak he once was, can be equally as impressive given he is par or superior to nearly every player mentioned from a technical point of view.
He's absolutely years away from being a spent force, imo. His biggest enemy (if injuries don't ruin him) will be his own motivations to play and push himself with so many games under his belt by that point in time. After such a long and demanding career, one wouldn't blame Messi for calling it quits before 35, damaged body or not. He can go on to have a strong career in its own right as a classic #10 or an Iniesta style hybrid for the next half decade if he so chooses. He'll still be comfortably contesting as one of the best players in the world in either role, too.