I've seen a common thread in many comments over the past week following the Nikon Z6/Z7 announcements, and I'm seeing it about the (rumored) upcoming Canon announcement.
"It's a new lens mount. I don't want to/shouldn't have to/won't use an adapter on my lenses"
One of the advantages of mirrorless cameras is that the lenses can be closer to the sensor. This, I understand, provides some advantages for the lens design. If this is true, then, in my opinion, Canon would be limiting their future capabilities if they release a new family of high-end mirrorless cameras that are compatible with EF lenses without any sort of adapter.
Building new lenses takes time. While it would be nice if Canon would release their new camera line with a full suite of 30 lenses available from day one, that's not likely to happen. Instead, I expect Canon to follow a similar path to Nikon, releasing two or three lenses with the camera, and announcing that a couple more will be available in the near future.
I'm old enough to remember when Canon introduced the EOS system in the late 1980s. We take it for granted today that there are dozens of different EF lenses available, with multiple options for every budget and every shooting scenario. But when the EOS system first came out, there were only a handful of lenses available initially. Many pros and serious amateurs held off switching to it precisely because they had large investments in FD glass, even though Canon sold a high quality (so I heard) adapter. It took a few years before there were enough EF lenses that the system reached critical mass and it became clear that this was where Canon was betting its future.
I expect the same will happen for a Canon mirrorless, and for the Nikon Z mount. It will take a while to fill out the lens lines. If the companies are really committed to these products, they will take a long-term view. They'll get there eventually.
(Incidentally, unlike Canon, Nikon's AF mount was natively compatible with many of their older MF lenses. Those of you who remember the old rec.photo usenet group(s) may recall that Nikon fans loved to rub this fact in the face of Canon fans well into the mid-1990s.)
Another often repeated trope is:
"Sony is too far ahead. Canon/Nikon will never catch up."
Sony definitely has a significant head start in mirrorless. I'm sure there will be many significant things that Sony does better than the initial Canon mirrorless, just like there are major things they do better than the Nikon Z6/Z7. But somebody will buy the Canons and Nikons. Maybe there will be one feature, however minor, that Canon and Nikon do better than Sony. Maybe they already have a couple of Canon/Nikon lenses and don't mind using an adapter. Maybe they just prefer the brand. Or the menu structure. Or the feel of it in their hands.
Right now Canon and Nikon are definitely playing catch-up in mirrorless. Sony has a huge head start and pretty much owns the market, but Canon and Nikon have a lot of experience making cameras and a lot of resources. If they expect to be number one in the market within six months of their initial product launch, they're doomed to failure. But if they're in it for the long haul, they have the potential to be true market leaders. It may take five years, maybe more. But they'll get there if they stay focused on it.
Hang on folks. It's about to get really interesting!