Are you not aware that WotC has insisted, time and time again, that it was the same edition? Saying "the 5e Core Rules don't work with it" is at a direct contradiction to that. There is no honest way to claim that it has "always" been an update to the core system, since those in the best know claimed otherwise.
Been following along all throughout the playtest, so yes, I am well aware of what WotC has said on the matter.
They're giving the 5e system an update. That's what they were always doing, and what they've always claimed they were doing. It's still 5e and they're designing it in such a way that older options (and in particular, adventures) should still be compatible (albeit with a bit of tweaking, perhaps), but the expectation has always been that most people will upgrade to the new system and that everything they publish moving forward will likewise be designed to use it.
There's nothing stopping you from continuing to use the '14 ruleset, and you can even try bringing in options from the '24 PHB and future books too. But they were not - and never were going to be - designed to be used in that way.
If someone provides you with a toolset alongside instructions on how to properly use it and you then try to make the toolset do something it wasn't designed for, it's not their fault if the tools happen to break down in the process.
Many of us strongly suspected that they were misleading the public, be it in a quest to not have sales drop off after the announcement or for some other reason, but this is the first actual confirmation that the earlier books will not get errata, which is how you update within the same edition. We've seen it, like them publishing the Triton several times and then going back and errata'ing every one of those books when they added Darkvision to them.
Errata makes sense when they're fixing a mistake or making minor tweaks, not for a full book overhaul.
They were never going to errata the entire text of the '14 PHB to change it to match the '24 PHB, and if they did, people would scream bloody murder about their '14 books being taken away.