I don't see any reason why an owner of an 850D / 90D / R10 would see an ultra wide less any less useful than an 5D / R5 owner.
Absolutely! I couldn’t agree more.
I was trying to be brief in my original posting, so perhaps my intent was unclear. I wasn’t trying to throw shade on anybody’s needs or uses.
Choosing a camera is an optimization process, a compromise, if you will. You try to find the one that best satisfies your needs and constraints. Usually there’s no one camera that’s perfect for every consideration, so you have to prioritize and make tradeoffs.
If you’re looking for a jack-of-all-trades camera, any modern camera, FF or crop, can do it. I’m sure Canon will migrate the EF-M 11-22 to RF-S before too long (not sure about 11-55).
But my point was that if you are primarily interested in ultrawide fields, FF might be a better choice. A 10 or 11 mm lens on crop is like a 16-18mm lens on FF. Very wide, for sure, but not like a 12, 10, or even 8 mm on a FF. If you’re happy with 10-11 mm on crop, that’s all that matters.
Of course there are always other considerations, size, budget, other features... It’s not a simple formula, and everybody’s priorities are different.