A large portion of reddit contributors use 3rd party apps. Will reddit even be worth using if those people leave? That’s a huge risk, and I’m not confident Reddit has fully evaluated the second order effects of this API change.
Most top-level reddit posts come from bots reposting old content. Those bots aren't using the first party app...
No idea how many of those apps are third party, but I would be willing to bet 'most' of them.