It's more common than you think. I've had Canon, Nikon, and now Sony as my main kit over the years. On the side I have also owned Minolta, Pentax, Contax N, Contax 645, and some Hasselblad stuff. Oh, and I had some Olympus mirrorless kit for a while, too. I like to try different things.
The fact that you personally have switched around doesn’t make it common. That’s another, even bigger trap the people on this forum fall into – they believe their own desires and choices represent those of the majority, when that’s rarely the case.
Market share changes slowly, installed base even more so. Most changes occur at the entry level, once you’re talking about full frame systems users are generally meaningfully invested in lenses, flashes, etc., and the inertia and more importantly, the cost to switch systems is very high.
You misunderstood my comment. I'm not saying people are going to immediately jump from the R3 or A9ii to the Z8. I'm simply saying those are the cameras that compete with the Z8 in today's market. They are the tier 2 cameras from each maker. Right now the Z8 is better spec'd overall than either of the other two #2 cameras, and that behooves Canon and Sony to release something better.
I understood, I just disagree. People on gearhead forums like this one like to compare cameras across brands. Some review sites mention comparisons to other brands, usually tangentially (DPR is/was an exception, but unlike most reviewers that earn money based on affiliate link sales or hawking LR plug-ins, DPR is owned by a major retailer, and manufacturers terms with retailers aren’t equal).
But most actual buyers of higher-end cameras have a bag full of lenses from Canon, Nikon or Sony. Most are not looking at the current models from each manufacturer and seriously considering which to buy. They’re mainly looking at the current offerings from the badge on their lenses and comparing that to the camera they have, deciding if now is the time to upgrade.
There are far, far more owners of Nikon D6xx, 7xx and 8xx bodies looking at the Z8 right now than owners of Canon or Sony bodies seriously considering switching systems.
I expect they will release the R1 next year but it's embarrassingly late. The A1ii is also likely to be released next year and the Z9ii won't be far behind.
Likewise, Canon’s main market for the R1 are current 1-series DSLR owners, secondarily owners of 5-series DSLRs and MILCs, then people who upgraded from one of those to an R3. At that level, owners of other brands are far enough down the list they have no meaningful impact on design priorities or timelines.