Yes, the team is carried by Suarez and Messi, I agree on that. However, I couldn't disagree more with the conclusions you make. It is a shame that such a talented squad has to be carried by those two players.
You claim Rakitic, Busquets, Alba and Vidal are finished. They are definitely playing like they are, true, but Busquets and Rakitic are 31, Vidal is 32 and Alba is 30. Meanwhile, 33 year old Modric won the Ballon D'Or playing in the exact same position as those guys do. And from those five, his style of play is most likely the most demanding speaking from a physical perspective. And those Barca players aren't just playing like seniors this season, they've already looked finished aged 28/29 as you yourself admitted regarding Busquets. So maybe they've just decreased quicker than anybody else in their position. Or maybe it is a matter of coaching?
However, even if you ignore those past-it players and factor out Messi, the team Valverde has at his disposal is absolutely elite. They just bought Griezmann and de Jong for not-so-small fortunes, Suarez scores like he wants, Arthur looked like Iniesta reincarnated last season. It's a shame that a team like that relies on Messi and Suarez to bail them out so heavily.
I also disagree that you can't fit those players into one squad. Yes, Griezmann is no left winger, but Messi is extremely versatile and can play as a false nine, second striker or playmaker. That would leave room for Griezmann on the right wing from where he could cut inside, having the ball on his strong foot. You could also play Messi as a 10 behind Suarez and Griezmann, like Sarri currently does with Cristiano, Dybala and Higuain. Just compare that to Pep, who sent Thierry fecking Henry to the left wing, played Fabregas as a false 10, turned Dani Alves into a player who covered RW and RB simultaneously and so forth.
Thing is, if you watch Valverde's Barca play it is painfully obvious that there's no identity, no shared game plan, no concept. They rely on individual quality so heavily. It's a collection of individual geniuses but together they are actually less than the sum of their parts. And that's down to Valverde. Under the right coach, this team would be lethal. And I'm not speaking of Guardiola or Klopp level of quality. Tuchel, Ten Haag, Sarri, etc. would already be enough to get much more out of them.