There may be some impact, come July, when the third party apps stop working. However, I have to imagine that the vast majority of mobile users use the official app. Quality may take a hit, with the loss of some mods and mod tools, but Reddit will be just fine. Sadly, Reddit rates too highly on content, users, and resultant utility (for many communities) for most users to completely abandon it.
Reddit rates too highly on content
But who provides the content? Power users. Reddit follows the same curve as most social media where only like 1-5% of the users actually post the content, and the rest are consumers. When the content creators are gone, it’s just a platform with no content.
The only people who will stick to submitting content are the poor content reposters or various spammers, which the mods have been doing free labor to filter out. Heck, even the bots using the API will die too, so all you’ll have is the TOS-breaking bots posting content.
This will not end well when third party apps are gone. I didn’t realize it myself, but most of my time is reading Reddit when I’m bored in bed, or on the train, on my phone. I’ve been a redditor for 17 years, and my time now has mostly shifted from my desktop to the “RIF” Android app, and without that, I’m simply not using Reddit, and have already uninstalled.