Because most people aren't going to know about the errata. They are simply going to play the game as written. As an additional reason, intent is not not always the best way to play something.
It's blatantly wrong to say that it's not how I should rule at the table, or how anyone should rule for that matter. Saying that people shouldn't rule that way is saying that people should never create a house rule, since those rules are contrary to design intent.
Feel free to house rule the ability to conform with the design intent, but you really shouldn't be claiming it as the One True Way like that.
Don't get me wrong, I'm not saying you shouldn't rule that way because it is contrary to design intent. I'm all for throwing the intent, and even the rules, right out the window if you've got a better idea. Make all the house rules you want, I sure as hell do.
No, I'm saying you shouldn't rule that way because it is taking a class strength and turning into a stupid nonsensical vulnerability, thereby killing a character based on a technicality. To kill a character based on a "literal" reading of a rule in clear, knowing contravention of the intent of that rule is asinine, and if you do it you should feel bad about it. Unless, as I said upthread, your players have signed up for a meat grinder experience and know ahead of time that you're going to kill their characters... then feel free to use "rocks fall, you die" rule interpretations.
I'll go one further, in fact: Jeremy Crawford was wrong to delete his tweet interpreting the rule according to its intent, and to post an "official" interpretation supporting the "literal reading." If the book came out with wording for a spell or class ability that contravenes the designer's intent, and that contravention will cause player characters to be killed in the most difficult to undo way they can be killed, then they should have issued errata to correct the rule in the book. It might not be such a big deal, really, since one might expect a DM to simply rule the way it was meant to work (that being the reasonable reading,) but no... there are clearly plenty of folks who get all exercised over the "literal" reading, reason be damned, so it should have been corrected.
I remember this argument from back when the thread was young, Max. Unless I'm very much mistaken, you admitted back then that you wouldn't rip up someone's character sheet over a technicality, a mis-statement of the intended rule. I really do not understand why you have anointed yourself the champion of the literal rule, but regardless... I have no hesitation in saying that the literal interpretation, as described by Jeremy Crawford, is bad wrong (un)fun.
(Unless you get all the players to agree to that sort of thing in advance.)