Arguably, Unity could survive without the game engine -- they just need to survive the consolidation of the mobile ad business. Many companies already used Unity Ads without using Unity engine, even moreso after the acquisition / near merger with Ironsource. That acquisition was entirely about staying a step ahead of the consolidation wave, and nothing to do with the engine.

Of course, while that idea may be arguable it's also absolutely idiotic. But I won't be at all surprised if Unity continues to steer in that direction, especially now that the Ironsource people comprise a large part of the company.