Let me preface this with the confession that I was the one person who supported the Super League.
But hot take and this in not a likely prediction or anything, but very much a hot take.
If a major change in the football landscape is incoming it's the fifa club world cup gaining relevance. I say this bc if Saudi Arabia can have 6 teams stacked with top players and a decent league around that you have another massive league, meanwhile Qatar and the MLS are attracting top players too.
If there can be 5 Non European leagues which have 2 teams that are UCL level an expansion would make sense.
But if that happens, quality players will be spread across even more leagues and the bottom half teams across the European clubs will get worse and worse, clubs like Werder Bremen, Rayo Vallecano, Empoli cannot compete at a decent level in such a world.
Best case scenario, the domestic season becomes an 18 game season with 10 teams in the top division with the fifa club world cup in whatever way its formatted being the primary revenue generator for top clubs.
This could make the UCL either another competition or it could make it the lower tier competition. Perhaps smth like the Euros even.
Domestic rivalries can exist in the second division too, just as they do now! And the top rivalries stay in the top flight. Betis and sevilla still play each other in the top flight, but The Asturian Derby with Real Oviedo vs Sporting Gijon is already in the second division and is one of Spains major rivalries.
I mean there's no catalan derby anyways with espanyol being relegated. Las Palmas and tenerife are in different divisions too.
Its just a restructure of who plays whom and how often, not the end of football clubs.