But this time we already have many of the actual changes already implemented, such as with the new Starter Set that they confirmed would stay in place through the revision. We have.the new Monster designs, and the new Race paradigm, already in play.
...
which again says nothing about changes they haven't revealed yet. Stuff like, as stated, the
almost guaranteed replacement of the Ranger with a new version and heavy rewrites of several PHB subclasses.
You cannot argue from the fact that we have seen certain changes in a certain pattern that it is somehow
impossible to get changes that
won't fit that pattern. Which is what I'm arguing, as I said before: I think the changes we've seen are the tip of the iceberg, and that there will be substantial changes to a lot of "behind the scenes" stuff, things that will be "completely compatible" with old material in the sense that you will still be able to run it, but not 100% fully backwards-compatible in the sense that it won't really be designed to support the old models.
For example, I expect encounter design to shift, I expect there to be rather more serious changes to monster design than we've already seen, and I expect there to be significant changes to expected rests. They can't implement those changes right now because it would be too dramatic a shift for classes still dependent on short rests. It will still be "compatible" in the sense that you
can still run "original 5e" characters through new-style adventures, it just will be much more swingy (and likely more punishing) for certain classes.
Which is why I said that these words can mean something rather weaker than it sounds like they mean. WotC has a vested interest in making people as enthusiastic and positive toward this change as they can. Thus, they have a vested interest in presenting it in as positive a light as possible, which often means playing up compatibility--not outright false claims, to be sure, but
massaging the truth wherever it is less convenient. And, like I said,
we have the playtest to demonstrate this already. It's not like this is a new pattern. D&D Next was billed as ultra-modular, capable of slotting in all sorts of highly divergent gameplay styles through "toggles" and such. The end product has a small selection of fairly weakly-supported optional rules, most of which just change the flavor or pace of things, without actually shifting the gameplay experience. The
exact words of their statements were equivocal, people read them pretty strongly to start off with and a portion were disappointed when that strong interpretation failed to materialize.
Be ready for that pattern to repeat itself. I obviously cannot provide any guarantee that it will happen. But it
has happened in the past.