I don't think this will be an issue when Messi retires from us. Nobody will be expected to fill Messi's huge shoes, but that player needs to be one of the best of the next generation of stars and that's why I think we should seize upon this opportunity that Neymar left for us and build up our team RIGHT NOW to prepare for the post-Messi Barca.
Admit it or not, having Messi has been a double edge sword for us in terms of attracting outside talents: For players/talents in non-attacking positions I think it is a huge plus, having the honor to play with Messi and Co. However, for players/talents in the attacking positions some of them might have doubts about whether they will get the center role in the attack with Messi still around. With talents/players like Mbappe and Dembele etc. who are not even 20, it shouldn't be an issue and we should make every effort to sign them.