That's maybe true as a first-order effect.
But, for the ads that everyone else sees to be worth anything, the site has to be worth visiting at all. If your most dedicated/prolific users mainly post/comment using third party apps, then making their experience worse will reduce the quality of the site overall (even if you start getting revenue on behalf of those dedicated users).
It strikes me as a very shortsighted move.
Twitter still maintains a critical mass of users and corporate accounts, but all the most talented creators (at least the ones I followed) have reduced their Twitter usage substantially or moved on completely.
Eventually new readers will stop showing up because there is no worthwhile content.
Thats really easy for Reddit to measure. Why are you assuming they haven't?
It strikes me as a very shortsighted move.
If you stop assuming that Reddit is run by idiots, and you consider the likely probability that they've modelled this stuff in some depth, it's easy to believe that your initial assumption is wrong, and that the users are on 1st party apps (or will be if others shut down), and that many will stay and continue to post rather than leave or stop posting.
Your premise is based wholly on the belief that you know more about Reddit users than Reddit does. That seems dubious to me.
I know that their UI and native application are absolutely bullshit.
For me, Reddit and Twitter are simply not worth visiting using mobile web or first party apps because the ratio of crap I don’t want to see (ads and promoted content) is too high vs the content I came to see.
YouTube is going the same way, I use multiple adblock rules to hide “shorts”, “people also searched for”, and “people also watched”. I pay for YouTube premium but feel like a mug for paying for a service which tries to force feed me content I don’t want. I’m on the brink of cancelling the subscription.
It's a well know historical fact that companies always do the smart move only based on hard facts.
Something that so many founders get wrong is the belief that something needs to be good to be valuable. It doesn't. It just needs to be better than not having it. That is often a tremendously low bar.