You're right. I'm one of them. I've been playing around with Mamiya medium format cameras since the mid 90's. Yet I've never sold a photo or made any money professionally. I just like taking photos and until recently medium format film has been the optimal choice for me (I lack the interest and patience of 8x10 or bigger.) But I think there's a difference between spending a few thousand on a camera versus tens of thousands.
Me too. Click my sig for (half of) the MF lenses I own (since I took the photo i've bought an(other) P6, a Kiev60, an(other) Zodiak 30mm fisheye, an(other) MC Flek 50/4, an MC Volna 80/2.8, an MC Biometar 120/2.8, an(other) MC Sonnar 180/2.8, an MC Sonnar 300/4, and for the hell of it, a P67 Takumar 300/4.
It started out as just a cheap way to use MF lenses as Tilt/Shifts on my 7D, then it kind of grew into its own obsession. And if I ever move to Mamiya 645 (which I was going to do soon but instead I blew my money on those MC Zeisses), they'll all adapt (and then when I win the lottery i'll get a digital back for it).
I know it'll never happen, but an EF-mount MILC with proper 44mm flange distance and no mirror, and a sensor maybe 40x30mm is highly possible, add in Hybrid Sensor AF and IS and it'll beat almost every current MF system for everything but IQ (well, maybe IQ, but hey, this is canon with their dirty read-noise ADCs we're talking about here).
Just consider image-circles for comparison:
24x36mm, image circle is 43.3mm.
30x40mm, image circle is 50mm.
30x45mm (Leica S2 size), image circle is 54mm
33x44mm (Leaf Credo 40 size), circle is 55mm.
Blow a FF sensor up to Leaf Credo 40 or Leica S2 size, and you're only pushing each corner 6mm out further from the centre, if even that.
So yes, some lenses (that already vignette heavily on FF) won't be too useful, and EF-s is just out.
But there's a lot that don't.
Check out the vignetting results at TDP, set the aperture to f/4 or so, and notice how a lot are only 1/3 stop or less at FF corners, blow that out only 6mm wider at each corner, you'd be less than 1-stop vignetting on a 33x44mm sensor. Like the TS-Es and 8-15 as already said. How about the 100 LIS or 180L Macros? Even the 50/1.2L at f/4 looks like it'll cover 55mm image circle easily, with no more vignetting than wide-open on FF. It sure beats an EF-s 15-85 at 15mm at any aperture on APS-C.
Yes, I'm only looking stopped down. But you don't run an MF lens at f/1.2. Hell, the fastest MF lenses ever (mass) produced were f/1.8-2.0, and most primes were f/2.8, zooms are f/4 or slower.
Don't say it can't happen.
Because it can.
Technically at least.
We know that it's Canon, and they probably won't, but that's a different argument.