Ronaldinho's peak occurred much earlier than Neymar's current age (27). Ronaldinho was at his best from 2002-2006. Reaching his peak (contrary to popular belief) in 2005 rather than the year after. At least individually.
Neymar is a great player, top 3 without a doubt IMO, and I do actually believe that he has at least 3-4 years left at the highest level (he could even possibly get better) but it requires a fundamental change from himself and that of his entourage. If he truly is desperate for a return and to take the mantle once Messi declines further or calls it a day.
The problem however is everything that I wrote a few days ago of negatives. Everything from how to fit him into the team with Griezmann likely arriving (almost guaranteed), Messi and Suárez going nowhere, what will happen with Dembélé, the club losing face, "club de amigos" getting further reinforced and running the show more than ever, no sports project or any long-term planning in sight, our wage bill reaching dangerous levels etc.
I would have less against a Neymar return if the useless board did more of a generational change and got rid of the likes of Suárez and Rakitic. Or if they had appointed an actual manager and not a yes-man and a "gestionario" which Valverde is. Nothing more and nothing less.
Griezmann and Neymar arriving obviously means the end of Dembélé. While it should mean the end of an almost 33 year old Suárez rather than one of the more promising attacking talents in the world in Dembélé. The misuse of that player has been criminal. Dembélé should not be parked on either the left or right but more close to the goal given his insane pace, ability to run behind the defense, him being ambipedal (a rare gift) etc. He could potentially turn into a Eto'o like footballer. The same Eto'o that started his career in Mallorca playing as a freaking right back initially. That would obviously require a top manager in the mould of Pep, Klopp, Pochettino etc. to develop that kind of talent not a clueless Valverde.
Neymar-Messi-Griezmann (Messi and Griezmann interchanging positions whenever needed, in fact all 3 could do that) with Dembélé being the first substitute of all 3, playing whenever one of the 3 would need a break and as often as possible. That would be a better solution than keeping an almost 33 year old Suárez (long past it) and selling a 22 year old Dembélé instead. Griezmann could at times play as the most offensive midfielder in front of DJ and Arthur and behind Messi with Neymar and Dembélé on the wings against weak opponents on paper at the Camp Nou. Or a 3-4-3 formation at times. Maybe not with Valverde around though. Not maybe, most certainly.
EDIT: To begin with, as written a few days ago, it makes zero sense to sign both Griezmann and Neymar. Signing one of them while getting rid of Suárez and replacing him with a Jovic or Lautaro kind of attacker (traditional number 9) would have been way better while keeping Dembélé but getting rid of Coutinho.