Re the last bit...Honestly, you're not wrong for my game or for me as a DM. I have no hesitation to take a rule from a different edition or a different game and adapt it to my 5e campaign, or to house rule something to work better for my group. There are other DMs for whom it seems to be a big deal, though, and I think the distinction needs to be preserved. Some DMs, especially new ones who haven't played other editions or other TTRPGs, really seem to feel as though they should cleave to the rules and not go off the reservation, and others (perhaps those involved in the Adventurer's League) are constrained by the terms of their particular game groups.
Another problem I see, and the thing that has kept me active in this "flogging a dead griffin" thread, is the emergence of an attitude that Jeremy's Sage Advice represents the "right way" to interpret the rules and play the game. There is no one right way that works for everyone, and I really think Jeremy's use of the "official" designation will cause more harm than good, creating the sort of confusion we're seeing in the post to which I was responding in the text you quoted.
Edit: Also, it is important to note that the SAC is not at all like "every rule in the PHB," because it doesn't contain any rules at all. It's all rulings, which we've clearly seen are subject to Jeremy changing his mind about.
A key thing for me is the not in a vacuum pov.
Sure, rulings by tweet by JEC have changed. So have actual rules by eratta. I sm not sure an SAC element tho has bern changed, but maybe so.
But the key is this... The tweet storms have on a few edge cases swapped around a bit - SM most notably.
But the SAC has not seen that much (if any) flippy floppy stuff. Thats seems to be cuz by the timrle it hits SAC its gone thru more review.
With the January 2019 SAC they changed it from JEC tweets official to only once it makes SAC is it official. Also, most all of the SM garbage was thrown out, no indivisible action, just timing. And the Hew official on official threw all those old tweets under a wagon.
They even have a section describing why have Sage Advice that kinda touches on your pointbof what benefit they think it will serve even in a rulings by gm world.
But, to me, your fervor is a bit far out in your insistance that SAC is so much to be just treated as advice and the thread line you keep skirting around for ruling vs rule etc.
For such narrow distinctions/divisions on status, it seems much stronger an invaldation than that narrow a division warrants.
I find it quite easy to see the SAC as the "finished" official rulings and the tweets by JEC as the wip previews, as they describe it more or less.