Messi is very much in his prime, last years messi is as good as he ever was, just slightly different.
Apart from that, being critical with Messi after that dramatic 2014 season where he was slowly walking around looking totally uninteresting and a shell of the guy he was before the injuries logically spark frustrated reactions by fan who know what he was capable of.
And to be honest it did look like a change of scenery was what he needed at the time and with the money Barca could have easily rebuild the team around Neymar in hope to revive past success.
It al seemed very logical back then, even until pre-anoeta despite everyone loving Messi and what he has done for Barca. Its easy to go al captain hindsight after a won treble