If Canon retired the EF-S mount, wouldn't that harm the upgrade path to EF lenses (particularly L series)? Would someone using an entry level body with EF-M mount really be as likely to buy an L lens if they have to buy an adapter, too?
I just don't see Canon dumping EF-S any time soon...
1) Agree with EF-S sticking around for a long time. There are currently (according to TDP) 14 first-party EF-S lenses available today to only (off the top of my head) 4 first-party EF-M lenses. So obsoleting the EF-S mount would put a large burden on EF-M glass development. I don't think they would do that in a leadership position in crop cameras unless crop sales were being devoured by mirrorless sales -- many folks have said that this is happening, but it has been in stops and starts. Crop camera sales aren't eroding from mirrorless like compacts are from cell phone camera use, so abandoning a mount that Canon can build lenses for cheaply in their sleep seems very premature.
2) Mirrorless enthusiasts absolutely
buy adapters to try all kinds of nutty lenses on their rigs -- full frame glass, ancient old lenses with manual focus, other companies' lenses, etc. But soccer moms and family archivist dads who buy a camera that happens to be mirrorless
will want native glass for that mount. So today, they can go sort-of-small with a crop camera that retains the ability to use EF glass natively, or you can crazy small with mirrorless at the cost of native lens connectivity without an adapter.
In many cases, going small and using native mirrorless lenses means waiting for the nicer FF lens you really want to be made for your mount
. This is the mirrorless quandary, and it should be no surprise that companies that lack as comprehensive lens offerings as Canon/Nikon (i.e. everyone) would rather throw the house at native mirrorless lens development rather than try to build an on-ramp for people to use larger existing lenses. As much as Canon owners with a dozen lenses love the value of an adapter for EOS-M (making it an easy 2nd/3rd body addition to our gear), we are very much in the minority. A mirrorless devotee who only uses one body wants great native glass for it, plain and simple.