. The way I would deal with "crop" sensor make it ff but set default as crop in stead of ff and use RF mount.
would not work in real life to replace crop-sensor DSLRs, because R-mount crop-sensor cameras would be "too large" compared to what is really needed and to competition. even more importantly, they would be too expensive and not sell well. xxD class class buyers are typically people on a limited gear budget, not willing or able to pay for FF-sensor camera body at or near current prices (north of 2k for body only).
furthermore, "firmware nerfing" FF cameras to "crop only mode" would inevitably lead to all sorts of "unlocking" attempts. folks would buy the somewhat less expensive "crop" model and run "Magic Lantern"-style firmware or any other such "hack" on it to unlock FF. not a good scenario for Canon ...
Also, with R mount on camera users could only buy expensive RF lenses (FF glass) as native lenses (in addition to using existing EF-/EF-S glass with adapter).
with EOS M crop-sensor cameras they get
* compact gear
* with very good IQ and very decent performance at least on par with xxD
* at much better prices (eg M50 half price of 80D, 1/4 the cost of EOS R)
* can use all existing EF/EF-S glass with adapter
* get compact, good and relatively inexpensive native EF-M glass
APS-C crop sensor cameras with R mount simply do not make sense. neither technically nor commercially. Not for Canon and not for customers. it ain't going to happen.
All Canon crop-sensor DSLRs will be replaced by EOS M system. i am also convinced they don't need to offer as many models with only slightly different features.
a lineup of only 4 well-defined EOS M bodies can easily cover the entire spectrum from
* xxxxD (lowest cost, entry level, EOS M100 successors, tiny size, last gen sensor and AF, no EVF, simple UI)
* to xxxD/xxD class = M50/M5 and successors,
* to 7D class = slightly bigger grip than M5, top-notch AF and fps, full weathersealing, decent battery (still smaller and far less expensive than any EOS R model).