I don't know, loosing biggest stars of league is big deal, but I wouldn't call it degradation. Offcourse, if Barca will continue to fall after Messi will leave, it will be a big problem not for club only, but for whole league too. Few years ago I was sure that best football is played in La Liga, especially when Spanish clubs completely dominated in UEFA competitions. But now I don't see any their advantages against EPL.
It is too soon to judge anything but I won't feel strange if it happens next 2 or 3 seasons. It happens for all major leagues in Europe, from Premier League to Serie A, Bundesliga and Ligue 1. In Ligue 1, do you remember what happened with Lyon? They dominated the league nearly one decade but now they won't be able to fight for the trophy last many seasons.
I think it is true, indeed the attraction between Ronaldo and Messi makes the Spanish League rating good, because they are two great players in two big clubs who have high competition. I once read an article that Ronaldo's departure to Juventus made the audience in the Spanish league smaller. When Messi left, I thought the Spanish League would lose its ten-year star in the Spanish league. But I think it's natural that every player has old age and the fans will lose them. I hope there will be a new star like Takefusa Kubo, I think he is like Messi.
Players have to grow from youth to super stars, then be older and lose their best forms and ratings. It is a cycle of life. Takefusa Kubo, I know he is from Japan. He and Takumi Minamino might be next proud of Asian players in Europe. They all have good forms in La Liga and Premier League.
Maybe they will be next Park Ji Sung, Son Heung Min, Hidetoshi Nakata, Shensuke Nakamura.