Before delving into specifics, I’ll reiterate that your claim that Canon ‘didn’t think through’ the parameters of the two MILC mounts and consider users moving between them is simply ludicrous. You may think they made the wrong decision, but if you really believe they didn’t consider interoperability in their mount designs, you’re living in an alternate reality.
All of your arguments are purely speculative, based on your opinion of what buyers want and what they would have done or will do. You’re totally ignoring the fact that Canon has mountains of data on what buyers actually did.
The M line is almost a decade old. The EOS R came out nearly 3 years ago, the more affordable EOS RP came out over two years ago. Canon knows with high accuracy how many EOS M line owners bought an EOS R line body. Likewise, they know how many APS-C and FF DSLR owner bought a FF MILC. They know how many APS-C DSLR owners bought FF DSLRs on which their EF-S lenses wouldn’t mount). They know how many and what types of lenses those people had before the upgrade to FF, and what lenses they bought subsequently. You…have an opinion. You can bring your opinion to a data fight, but you’re not going to win.
Personally, I suspect only a tiny fraction of APS-C owners upgrade to FF. I suspect most of Canon’s FF MILC sales are to people who owned DSLRs (and their lenses are easily adapted).
People are dying to buy lenses? If so, an incompatible mount means Canon sells one more lens.
People who can’t afford a FF MILC lens aren’t going to buy the body either? Well, so what? First of all, Canon wants serial customers for their high end gear (any FF setup is high end). The loss of someone who can’t afford the kit lens isn’t a big loss in that context. Second, those people could buy the body and an RF 50/1.8 for relatively little additional outlay.
Using any lens on any body is a selling point? Nikon DSLRs have that ‘very attractive sales point’ and Canon DSLRs don’t. Who has sold more? Clearly, THAT didn’t sell (a perfect example of data trumping your opinion).
Unifying the mounts would be the tail wagging the dog. Your suggestion that the EF-M mount could have been bigger to match future RF would mean bigger M bodies and bigger M lenses. Both contravene a major selling point of the M line. Canon should compromise the M line for the relatively few people who will upgrade to FF? That’s silly. Clearly the M line is a success, Canon’s decisions were spot on there.
Conversely, making the RF mount match the EF-M mount would constrain the R optics. Canon wisely chose to optimize both mounts for their intended markets, armed with the historical sales data to understand the consequences (presumably not significant) of those choices.