Surely Mbappe won't be happy with this with how huge his ego is ?
No, but he still has major role in the team. Wouldn't be surprised if he's the leading scorer despite not getting as many dead ball situations and the attack not flowing through him as much anymore.
Messi and Neymar at this stage in their career are positionless players and lack pace. This could cause problems tactically in and of itself, but I assume they will figure out how to play off eachother and create good synergy again. Mbappe will be asked to play off ball more and use his pace to make runs in behind (a la Neymar 14/15). With two world class playmakers behind him he should be banging the goals in left, right and centre.
This is a great opportunity for him. Of course he doesn't seem all that intelligent and could throw a tantrum and refuse to cooperate.