You just need to look back to his last season with Ajax to see his value. I was genuinely excited about the signing, and felt he was exactly the sort of player Barca needed. A young dynamic midfielder with the ability to beat a man and run into space. With fast-paced wingers Dembele) and intelligent forwards (Messi) the space would open up and so should scoring opportunities. Unfortunately Barca's first thought these days upon regaining possession is to hold and wait for support. Messi is an issue in this regard as he's not capable of playing a full game of counter-attack football.
OK imo Liverpool, Munich and Madrid have been the most successful teams of the past 5/6 years. All three have succeeded through team chemsitry and in the latter's case some pure rabbit out a hat wizaerdry (with abit of luck). The first two rely mainly on a form of intense in the opposition's half pressure together with pacy wing hit the byline to stretch the play football. Thee really insn't the possession-based play seen at Barca. Madrid is a hybrid of both, or they certainly were until CR left.
What Barca failed to understand was that they either had to go back to the tiki-taka high intensity pressing that defined them or they had to continue with the success Luis Enrique eventualy fostered. They did neither! The midfield deteriorated, the defence deteriorated and and the attack fell off a cliff (neymar left and Suarez just got shit). It was the failure to get the midfield right that got the snowball rolling toward the absymal position Barca are in now. Players like FjJ should hav ebeen introduced in 2015. Selling Tiago was just plain mental. Integrating Academy players was a necessity but rrely happened. Still doesn't, and now that they're in danger of missing out on top4 it's even more unlikely.
Point is that if you're still going to build around Messi into his 30s you had to bring in the technical players that could work with him. It might have taken a couple of years for them to get up to speed but they would need to press as well. The Board didn't they went for young experienced speed, AND neglected the most important part of teh pitch, the midfield! Messi was never going to sustain the 2015 style unless he was able to remain further up the pitch. He hasn't been able to remain further up the pitch because the MIDFIELD has never been good enough since that last treble-winning season.
I've rambled a bit here, I'll stop.
tl;dr Recruitment has been shit, there has been no real defined style, it's been cobbled together around goat-level Messi. Messi has papered over teh cracks, until now!!