Don't believe that. Serie A teams at some point just started buying and playing old stars instead of young players with potential. La Liga is different. Even without Messi and Ronaldo, there's more than enough star potential in the league. Look at Madrid's squad, there are a few true gems in there. And they still have Hazard who's yet to stay fit for a sustained period of time for them. And then there's Atletico with Joao Felix. Even Barca, despite all there problems, have a few very promising talents in de Jong, Fati and Puig.
The Seria A decline is different from La Liga situation for sure so not comparable. They had their high before TV money got crazy and had reliance on rich national patrons for growth (Berlusconi, De Laurentis, Agnelli family, Ferrero, Moratti etc) the global depressions and high tax pressure hit their patrons very hard in the wallet. Unwillingness to invest further private money and to some extent a cultural stubbornness to adapt to the modern game.
Translating to lost Europa spots, fading star quality, stigma of calciopoli, old arenas that monetized fans very poorly with low attendance outside top clubs and derbies. Declining youth programs hampering natural regrowth didn't help either.
The old players fetisch was probably a symptom on part due to having to keep what you have past expiration date since you couldn't replace and cultural
part with untouchable status to geriatric champs aligning poorly with the more physical faster modern game. Compared to say Real and Varca that very pragmatically offed their older icons. They were overall late to react and there are many lessons to be learned from Serie A.
In light of all above Spanish and EPL clubs monetized the game better, modernized arenas and tactics better and increased buying power and consistency, excerting stronger pull on star players. Overall La Liga clubs are in a much better shape today and keep producing great players. They will rebound faster most likely since the core foundations are healthy except the unbalanced TV money.
Serie A with Juve and now Inter and growing list of more clubs to follow have finally modernized, monetized and tapped into international money and seem to attract a lot of quality players again.
As for the Spanish big 3 and current players, remains to be seen how much star quality they will have on their own without the Goat's. Perhaps a EURO and World Cup or a UCL win will energize some of them to super star status.
Atletico I have no clue about but Real are no doubt in a much better place than Barca given they acted earlier and more pragmatic to the shift and have soften the blow a lot. Barca going the Galactico route seem to have lost some of their identity. Might be we see a more normalised La Liga with much less star appeal. But not the full collapse like Serie A for sure.