I kinda explained why... In terms of ROI, it's an easy win. And let's not assume everybody is pleased with the R5. I'm not. And there are many more people involved in EF lenses.
Just because the parts all exist separately doesn't mean they can bring them together in a body at a reasonable cost. There is no mirror box part short of the 1DX3 that supports the rates the 5D has. We have no idea how expensive the "grid" PDAF sensor from that camera is, how much it relies on the more expensive RGB metering sensor, etc. These "already done" parts
You can't just assume ROI. And 5D4s didn't have to compete with a mirrorless camera. Is there production volume to support a 5D5 in a world where it's competing with an R5? Other considerations, assume the R5 is going to sell at capacity for 6 months. Where would canon rather put their supply-limited sensors? The R5 is pushing a new lens mount, which is strategically important.
Personally I'd love to see canon release:
- 5D5
- R5s (studio, high MP)
- R5c (active cooling, Dual CFexpress, more codec support, full size HDMI)
- Some crop/M cameras that have the new "Digic x" driven feature set
- An R1!
I think we'd all love to see Canon release a canon for ever use, but we're in a shrinking market, heavily supply constraint, with potential huge economic headwinds. You can't just say "The ROI is there!" even if we all want it to be the case.