I agree that you won’t get consensus, but WotC does not need that. There is a huge difference between 5% considering it incompatible and 80% doing so. WotC imo wants to keep that number small enough that people make the switch rather than staying with 5e or switching to something else entirely
I'm not saying that 80% will find it compatible or incompatible. I'm saying that most people have neither the time nor inclination to do the adjustments needed to make 5e and One compatible.
Take Level Up. It's supposed to be completely compatible. There are still people who ask "has such-and-such a monster been converted from 5e to LU?" when the only mechanical differences between a 5e monster and a LU monster is that LU monsters don't have alignments and have a "maneuver DC" which is literally 8 + PB + Str or Dex mod. And while martial maneuvers are an LU thing, there are also basic maneuvers, like grappling and shoving, which have always been part of 5e. Sure, LU monsters have extensive changes in
lore--for instance, LU harpies lack the "cursed elf" backstory--but the monster stats themselves are completely interchangeable. I've used them in 5e games without a single problem. It's why, while I'm converting monsters from D&D to LU over on that forum, I'm converting monsters from AD&D and, eventually, 3e, which are
definitely not compatible without extensive work; I'm not going to convert 5e monsters.
So what I'm saying is that even a small change will be enough for probably most people to completely disregard 5e books in favor of One. And right now, One has several non-small changes and we've only seen two playtests.