Without Buick, Cadillac can move down a level to the entry-level luxury market and cover most of what Buick did.Where I live I know a couple of people that own Buick crossovers. They would never buy Chevys. That may not make much sense, but there you go. I doubt they want to pay the $$ for a Caddy. These customers would be lost to GM if they closed down Buick.
Cadillac would have more of a mandate to create cars in the "luxury and near luxury" space. They could have a true Lexus ES or UX or NX competitor.
But Buick exists, so Cadillac doesn't need to cover that part of the market.