That said canon kicked all its customers where it hurts when they replaced their mount. Life is always weighing up one risk against another
As an earlier poster pointed out, mounts have little to do with mirrorless.
If anything, making a mirrorless system makes your mount MORE universal than before, due to the simplicity of manufacturing adapters between any camera brands when you have a whopping 20mm or so clearance to work with. So you can still use the old mount (meaning it's not a replacement or obsolescence),
plus the new ones,
plus a bunch of other companies' lenses you couldn't use before.
Nikon mirrorless cameras, for instance, can use canon DSLR lenses, whereas nikon DSLRs can't adapt canon DSLR lenses. Making the Nikon mount MORE flexible in mirrorless.
In fact, 20mm is such a huge distance that it also becomes trivially easy to make those same adapters do things like add
tilt shift capability, or add in a
manual aperture, to make lenses from other companies more useful (since you lose electronic communication when going cross brand)
here's an example for sale right now of an EOS lens to 4/3 body adapter with built in aperture:
Rakuten.com - AST Canon EOS lens to Micro 4/3 Adapter Build-in Aperture
And here's an example for sale right now of a Nikon F lens to 4/3 body adapter with tilt shift:
Kipon Tilt Shift Adapter for Nikon F Lens to Micro 4 3 Mount E P3 PL5 GF5 GX1 | eBay