There is a slight difference between real madrid who lost ronaldo and also barcelona which messi left at this time, when ronaldo left madrid, at that time madrid condition was also a bit limp but fortunately they still have benzema who can replace ronaldo position well, while barcelona is quite long relying on messi, unable to find a suitable replacement when messi moved to PSG, and even fati and depay who are expected to be the next messi in barcelona have not been able to show their best game at the moment, besides that the club financial condition is currently in trouble, adding to the condition Barcelona is getting worse, and it's clear that Barcelona's current condition is worse than when Madrid lost Ronaldo in 2018.
Benzema didn't replace Ronaldo all that very well, and nobody in the world could ever be the "next Messi" neither, these are two players that nobody could ever replace because between them they have like 12 or something Ballon D'or awards and that's more than any other two people combined. The real difference was that Real had a better 11 than what Barca had, they both managed to not replace their missing superstars but at least Real managed to put up a better 11 than what Barcelona could offered.
We also have the fact that Barcelona is doing horribly on finances as well, they have so much debt that paying it would take years for them alone and then they need to also get more players and pay salaries and so forth. All in all Real finished it with a lot better result when Ronaldo left, but when Messi left Barcelona still wasn't ready for this period.
I think you misunderstood what I said about
replacing the position, I didn't say in terms of replacing the quality of a player, and I also agree that no other player can match or replace the two best players so far in terms of quality, but what I mean is that Benzema is more able to replace Ronaldo
role or Task at Madrid in terms of scoring goals, you can see after losing Ronaldo, Benzema was able to transform into Madrid goal machine and it was proven when Benzema could be Madrid top scorer right now, but it was different with Barcelona which In fact we don't have a player who can replace Messi in terms of scoring goals, I don't even see a Barcelona forward who stands out in terms of scoring goals this season.