How do you come to this conclusion when the changes would be based on overwhelming player feedback?
I understand editions happen. I've seen it in other games. D&D actually tends to be less frequent than some. I'm not railing against moving to a new edition with cleaned up rules -- though I'd like changes to be more of the order of 1E to 2E or 3E to 3.5E, not the sweeping rewrites of 3E to 4E or 4E to 5E (although I like 5E better than either 3E or 4E).
I also think that getting player feedback is good. I just don't like the "living system" concept. An edition is based on the core books, especially the PHB. That needs to be solid enough, before release, to last the duration. I'd be okay with a reprint with some clarifications -- genuine clarifications, not changes; I believe they did that in 2E, though I don't remember for sure.
Changes to the core rules at a sufficient level to warrant a new PHB is pretty much the definition of a new edition. To "errata" things to the point that the original PHB doesn't actual contain what you need for RAW play at a table is indicative of a mentality that's more house-rules or playtest focused, not creating a stable, approachable product.
If I create a new subsystem for descriptive critical hits (to pick a classic) or post an Eberron conversion to 5E, there's quite a bit of leeway for me to come back and tweak things. Those are table rules that I'm sharing with other tables, one hobbyist to another. Do as you will.
If I were to get officially licensed to write the Eberron conversion, there would be a higher expectation of quality. If I find out that the printer didn't include the sentence about "recharges after a long rest" or I omitted it, thinking it was implicit, then it's "errata" for me to post a clarification and/or fix it in future printings. It's
not okay for me to decide later that it's underpowered and officially change it to scale with level (or whatever). That wasn't part of the rule when written. I can certainly put it in some sort of "recommended changes" or even a FAQ, but the published rule is still the published rule for the duration of the edition.
That's not treating it as a published work. Books aren't video games. They don't get patches or DLC. The closest you get are additional source books, which are additive, not replacements. It's a restriction of the medium and a professional lives with it.
Is there a threshold? Sure. If they officially change one or two "recharge with long rest" to "recharge with short rest", it's not a big deal. But, they should feel bad about having to do it. Opening the door, up front, or swinging it wide isn't cool; it's unprofessional. It isn't "responsive"; it's sloppy. It pretty much invites bad game design because "hey, we'll just errata it, if it doesn't work".
I have no problem with table rules -- I actually expect them. I have no problem if Mearls has his own table rules, or even if he posts them somewhere. I have a problem when the rules to the game are explicitly and officially fluid. Treat the product I bought as though it's a business or I won't be buying product from you. I might drop a couple bucks in your tip jar, but you're just another DM posting some of your ideas for people to read and you've lost your authority as "the definitive source".