That's calculating value against not having LLMs and current competitors. If they stopped improving but their competitors didn't, then the question would be the incremental cost of Claude (financial, adjusted for switching costs, etc) against the incremental advantage against the next best competitor that did continue improving. Lock in is going to be hard to accomplish around a product that has success defined by its generalizability and adaptability.

Basically, they can stop investing in research either when 1) the tech matures and everyone is out of ideas or 2) they have monopoly power from either market power or oracle style enterprise lock in or something. Otherwise they'll fall behind and you won't have any reason to pay for it anymore. Fun thing about "perfect" competition is that everyone competes their profits to zero