The assumption that the apps would raise wages if people suddenly stopped tipping seems misguided. It's possible that people would just work longer and harder for the cash because they're desperate.
If they're that desperate, the company could already lower the wages.
Wages are at the legal minimum.
There is always hysteresis. People don’t just operate on some smooth indifference curve. You cant lower wages (due to past wage history), but total comp can go down (due to less tipping and companies not increasing wages). Furthermore, lots of gig/restaurant workers operate close to minimal wage, which further drives wage inelasticity (eg companies cannot drive down their wage).