5/10
Messi(30)
Suarez(30)
Busquets(29)
Iniesta(33)
Pique(30)
Rakitic(29)
6 out of 10 outfield starting players will be way past their prime within 3 seasons.
Umtiti, Dembélé and Coutinho is a promising start to replace what we have but nowhere near enough.
We just bought replacement for Iniesta (Cou) and hopefully Mina will prove to be good enough to eventually replace Pique down the road but even if he won't we still have some time to find his replacement. I feel confident in Robert when it comes to finding CBs.
If we'll really bring Grieizmann in summer then we'll have Suarez replacement for the next 4-5 years and in the meantime we can find someone younger, maybe even Abel Ruiz will develop good enough.
Rakitic is done anyway and shouldn't be that hard to replace him (just using barcaforum narrative, not my personal opinion about him
). Most people think Roberto should easily take over him (I don't). But overall I still believe he'll be the easiest to replace from the players you mentioned.
Messi can't be replaced by a single player so we shouldn't look for his direct replacement but rather try to make strong enough team until he's still here at his best and hopefully build our next core around MATS, Umtiti, Roberto, Cou, Dembele,... to take over in that transition period when he'll leave/retire.
So I think Busi will be the hardest one to replace but we still have some time to find his longterm successor.
Overall I feel much better for our midterm future than I did 2-3 years ago after we've signed Umtiti, Semedo, Cou, Dembele (though the later three will still have to prove themselves) and after seeing how MATS and Roberto have developed during that time.