Nonsense - it would be too labor intensive to modify the lenses and would probably cost more than buying the correct mount.
Assuming that was in response to me, you're missing my whole point. Canon has only three lenses in the correct mount: an 11–22mm zoom, an 18–55mm zoom, and a 22mm prime. That's it. So you can't buy long lenses in the correct mount
for mirrorless Canon cameras, and long lenses are where the weight of the lens causes the most sagging, and thus where I'd expect the maximum amount of IQ loss from the adapter.
It also doesn't help that the focus ranges they offer in EF-M are harder to work with than what is available in the EF-S and EF worlds, either. There's nothing with as long a range as the 15–85 (crop) or 24–105 (FF), and there's nothing as wide as the 10–22, period. Worse, the primary zoom lens is equivalent to about 29–88, and most people would probably find that to not be wide enough, resulting in a much higher rate of lens changes compared with EF or EF-S, where 24/15 is the starting point for most of the popular zooms.
BTW, the effort to convert a lens should be pretty negligible. For lighter lenses, they could probably do it just by making their existing adapter slightly longer, removing the metal lens mount (four screws, typically) and putting the modified adapter in its place. For heavier lenses, they would probably need to replace the top part of the body with a larger piece of metal in order to prevent excessive sagging. However, IIRC, that enclosure piece is usually easy to remove without fully disassembling the lens, making this also a trivial modification.
For the electrical connections, they could either clip the contact block from the current adapter tube to the contact block on the lens or they could build a blank circuit board to put in place of the existing lens board, with the appropriate connectors for the various ribbon cables on one side and extension ribbon cables on the other side to connect up to the lens's original circuit board, then mount the original circuit board at the top of the extension piece, though that would require slightly different versions of the mount adapter, depending on where the circuit board's screw holes are, so that would be a more appropriate choice only for the longer lenses, where you have to build a custom body piece anyway.
Either way, we're talking about maybe an afternoon's worth of engineering effort per lens, and about a 5 minute modification with fairly inexpensive-to-manufacture parts. Heck, a third party could even do this; it's that straightforward.