The problem is that Spirit is but going to survive 2024. Between their debt situation and a number of their A320 Neos about to be grounded thanks to a Pratt and Whitney eff-up, they won't be around long enough to keep anything competitive.
An analogy would be two competing gas stations at a very busy intersection with no plots nearby to build more stations. Just by existing they either drive each other down on price or collude on it. One going under means a new station could exist, but one buying the other guarantees prices do not go down.