I was stunned watching Messi in the ties with PSG and Juventus last season. He was genuinely terrible in all 4 games. I didn't watch him in La Liga but he definitely looks like he's lost at least a step to me.
I think even if Messi adapts to being that bit less explosive - and he will because he's the best player I've ever seen - Barcelona will still lose a huge amount of quality when Neymar departs. He's 5 times as good as Dybala and 10 times as good Dembele.
Overall though I think the quality of their team just gets worse with every passing year.
Neymar won't catapult anyone straight to the top - it is a team game afterall and PSG lack quality elsewhere - but he is utterly brilliant and on a completely different level to everyone they've had playing for them since the Qatari's took over.