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.
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.