The debate over third-party franchises in MTG has raged for years now, but the issue just came to a head with head designer Mark Rosewater suggesting that those who don't like Universes Beyond should either adapt or leave the game behind.
In response to a fan expressing concern over how Rosewater responded to previous complaints about Universes Beyond, the designer writes on tumblr that there are only three paths forward for fans of the best card games: they must either accept the change, find like-minded players who don't want to use crossover sets, or "walk away" from MTG altogether.