Some of these work, but a couple don't, because this isn't the first time they've been true.
That's historically never been an impediment. People jumped into 3E and 5E the very first second they could (in fact, S&S brought out the Creature Catalog before 3E's own Monster Manual). Newness to the rules has never been a barrier to third party support.
As above. Didn't stop anybody with 3E, 4E, 5E, and PF1.
Almost the exact same license as for PF1, and not dissimilar to the d20 STL. Neither of those slowed people one jot.
If any of those reasons were true now, they would have been true before. That means, logically (assuming we accept the premise that 3PP support is lacking) it's something else. It's something else that's changed.