What are you basing this on? Do we have any data regarding the percentages of mobile/old/app users?

I wish I still had the graph, but someone posted something for their (150k subs) subreddit recently, and it was below 10% old. No idea what sub, or what the source was so, so take this with a pitcher of salt.

It's about 5%. Not much variation by sub, even the technical ones are in that range, maybe a few % higher.

It’s not just about users, it’s about principles.

I have no doubt some users will leave, but I’m not sure what principles have to do with whether or not it would be the end of Reddit. Unless you mean in some subjective, personal sense.

Think of old.reddit.com as a canary. If it goes, then you know some shit is going down and Reddit will never truly be the same going forward. Even if you don’t use it, that should be the signal to leave if you haven’t already.

I would be genuinely shocked if old.reddit.com users made up a non-trivial amount of the audience.

I find tech/power-user crowds like that on HN tend to dramatically overestimate the proportion of a userbase they actually make up.