I think switching Messi with say someone like Sancho (just an example) would change how the team looks. We'd lose the genius actions Messi brings, but we'd gain in other areas which are very important. Work rate, pressing, defensive coverage, movement. All things we can all agree we suck at (Messi and Suarez are the main culprits in these areas).
We won't switch Messi for one player and succeed, that won't happen. He is still our top goal scorer and creator.
There will be a need to replace so many players in both midfield and defense, attack too most likely.
We are unlikely to produce a team that is elite and true CL candidate with him being a tactical burden, but we surely can win the league with him which is huge success during a retooling phase. Despite the fact that our fans has some shitty attitude toward winning domestically and has been taking it for granted.
Right now, who beside Messi need replacement?
Old players: Pique (despite being good), Alba (despite being hard to replace) , Busquets, Raki, Vidal & Suarez. All those players are on decline for few years (except Alba),
Dembele too as he is too injury prone.
If we can have next year with a young CB that can replace Pique, Young midfielder beside FDJ and Pjanic, add an attacker to fill one of the two positions beside Messi, we will then have enough tools to compete for a strong Liga campaign.
Next year add couple of more pieces and we are even better.
When the time comes and we need to replace Messi, we will be one/two piece away from achieving that.
The main problem, is that we have actually started retooling in 2017 and we are just failing in adding the right pieces. Arthur, Coutinho,Griezmann, Dembele,Malcom all failed for whatever reasons. Lautaro could be another name to add to the list. Umtiti injury fucked us badly
I like Trincao/Pedri/Puig/Fati/Moriba. But history suggest that we might be up for a dissapointment.
TLDR: We need to replace the old core while Messi is here, that won't achieve an elite team status but will be good enough for Liga, which is fine atm