Messi was brilliant in the UCL for Barcelona but you can't forget Dinho's impact on that team. Messi has 4 UCL's but he was a crucial player in 3 of those tournaments. Ronaldinho Gaucho was the player who truly revived Barcelona, similar to Cantona at Man Utd.
Ronaldo's record in domestic leagues is inferior to Messi's but if you look at the context it isn't that much inferior. Messi has 12 domestic league titles vs 7 of Ronaldo. But, he was pretty much a passenger in his first two La Liga campaigns, so it's around
10 campaigns where he was a key figure.
And two of them came in Ligue 1. In reality Mbappe was a more important player than Messi there and it's Ligue 1. I think that majority of people would agree that Serie A, La Liga and EPL are the best leagues in the world and Ligue 1 lags behind those three.
So it's around 10 vs 7 league titles for Messi and 8-7 if you exclude Ligue 1. To be fair to Messi he has far more golden boots, so he's clearly better in domestic club football but Ronaldo's adaption and flexibility is seriously underrated. He's only player to win the golden boot in Serie A, La Liga and EPL. Also he's the only player to have over 100 goals in La Liga and EPL.
And you said that Madrid won 2 UCLs after Ronaldo left, well Barcelona won 2 league titles in 4 years (after Messi left) And this Barca squad is arguably the best one since 14/15 season. So they are hardly falling apart.
You know who is falling apart? Juve and Man Utd where he was deemed as a "problem". So it goes like this:
After Messi left Barca- roughly remained the same
After Messi left PSG- they're even better now
After Ronaldo left Madrid- roughly the same
After he left Juve- Juve became much worse
After he left Utd- United is in much worse state compared to his second stint at Utd.
Leaving your bias aside. That I'll just leave it there because since some time you have board that boat to never leave it.
This time, for me (I might be wrong) what Kwabs meant, it's that in a historical point of view, Messi changed the scenery in Barcelona in a historical way that even as big as Barca would mantein to be and already was, it was crucial, marking an Era.
This (beyond Messi) is actually a characteristic of how Barca historically has been. A tad more messianic in terms of the influence of certain players in their history and how they operated usually in the market. From huge influences all over the place a la Cryuff (even if him as a player didn't brought the
CL), to sometimes for shorter periods (Ronaldinho (in a lesser way Rivaldo/Romario) and finally sometimes failing to build around and mantein a possible one (Maradona, R9), they historically had a tendency to seek a savior and at times finding them.
Madrid just at the beggining with Di Stefano, was quite messianic, yet later on they were always more flexible in their approach, in their transfers (at times at the cost of their very own academy) policies, in not needing as much as Barca some individual talisman to get bigger heights, at times even buying everyone out there, yet they learned how to operate as a huge club (or THE CLUB) after Alfredo and never left that boat, navigating some big storm at times, but being way more cynical, cunning overall and way less auto destructive at times knowing they already are THE club and Barca still the aspirational contender.
Only in later years Barca with Pep starting an almost full La Masia approach, Barca has finally settle in a more youth oriented team and even trying to lure their next Talisman from very young ages a la Messi (like Yamal now) instead of going and buying it. While also adding this sort of individual oriented huge talent to thrive more in their more collective complementary passing style academy than in the past.
That's why with Messi they had a period with his name written in stone, it's in their nature and they might finally find the
cure if Yamal ends being even greater and with their international scout approach snatching these types from all over the world very young (plus those special ones from the City itself or near) into a very well oild academy like it seems to be nowadays.
We'll see, not long ago with money issues, they had to pull out the palancas to not fall from a cliff and find the Lewas and cia to at least gain time for a Flick to get stable and develope their new young Army.