Steve I am really trying to understand what you are saying. f/1.4 is very fast. The Tamron you mention is anything but fast and is fat compared to the M line. So I don't see how that is any different than using a "fat" Canon lens. In fact, at 300mm the Tamron is very slow. You could always get a Canon 400mm f/5.6L.It is possible to make fast lenses with a small diameter--if they are short focal length as well! And the EF-M 32 is f/1.4 (not earthshatteringly fast, but fast). But any sort of long zoom (or long prime) is going to be slow--very slow--if they insist on making it skinny in outside diameter. I own EF primes that are reasonably fast, but they are ALL fatter than the EF-M lenses.
The point I'm trying to make is not completely incompatible with yours--yours is correct and prior to mine. Canon COULD make fatter lenses, that would be faster (or longer). But they'd have to want to make a new M lens in the first place and don't see a market for it (or enough of one to justify moving people off RF development) (your point, I believe).
But what would happen if they DID decide to throw some love in the EF-M direction? Would they finally break down and decide--"let's make a fatter lens so it doesn't have to be so slow"? Or would they stick with a restriction that has no <I>engineering</I> (or even marketing) reason that I can see?
The Canon rep I spoke to a few weeks ago was visibly annoyed by my Tamron 18-200, but if he doesn't like it--Canon can damned well give me an alternative (if they get over their skinny fetish). If Canon doesn't, he's got no real right to complain.
Anyway, I suspect most people don't want a big lens on the M. Tamron or an adapted EF or EF-s) must fill the niche for those that do. So I don't think Canon cares about the niche... or feels the EF, EFs lines fill it.