But at least you could use the same 100-400 or 100mm macro or even the 500 F4 if you wanted on a 7D or a Rebel, because IQ was good enough. And many just did that. The 7D with the EF 100-400 was one of the most popular combos.
Not possible with the M mount and making a $2000 M mount camera for $200 plastic lenses is not a solution.
For an APS-C RF camera you will automatically have 15+ RF already working natively. All Canon needs 4-5 APS-C specific lenses, like a standard zoom, wide angle and a few small primes.
I disagree with almost every sentence.
500/4 on a Rebel?? I don't think so. People who buy 500/4 do not use Rebel. 7D2 maybe, but even that is kind of obsolete now. R5 takes 45MP@20FPS, pretty much kills 7D2 (20MP@10FPS). Even R6 (20MP@20FPS) and R (30MP@8FPS) are competitive.
Sport photographers using all the huge white lenses use 1D series with just 20MP and still often crop a considerable amount. Current and upcoming FF cameras have reached the point where there's no gap to be filled by 7D series. In 2014 the 7D2 made a difference. Today, it (or it's equivalent around 33MP@20FPS) would not.
Also do not forget, APS-C might help on the long end, but has lot's of drawbacks (short end, noise, bokeh, etc.), it has always been a compromise ever since the birth of APS-C. We do not use it because it is good for photography, just for economic considerations.
But I think you didn't get the whole point. When using FF lenses on an APS-C body, you pay for and carry the weight of lenses which were designed to produce images on more than 2.5X(!!) size sensors. And you consider it to be totally OK. Anything but. Purchasing and carrying a line of FF L lenses and then pairing them with some cheap APS-C body which will not even really lighten the overall weight of the setup makes no sense to me. Yes there were times....I did it as well...early 2000s starting with 6MP 60D when there were no FF alternatives. The world has changed at 5D2 and now again at R5.
I think we kind of arrived where APS-C is paired with non-L glass and FF with L glass. Mixing APS-C and L glass is pretty much over. Possible but no point really.
Ah yes, and APS-C body is not even going to be effectively smaller than an RP.
The only thing that could make a huge difference if there were professional-grade APS-C lenses (70-200/2.8, 100-500, etc.). That has never happened and never will.