"m." subdomains are just a totally outdated concept and I wish they would disappear entirely. It is trivial for the web server to detect what device type, OS, screen size etc. the user is on and serve content accordingly.
Yes. Some might argue otherwise, but I'm with you there except…
… sometimes the mobile version of a page is preferable as it cuts out a load of crap including adverts and other distractions, just giving you the information actually being linked to. So "m." type links or other "force mobile view" options can be accidentally useful for a purpose beyond what their designer intended.
> It is trivial for the web server to detect what device [properties]
No, it is not trivial to do this reliably server-side with no client-side assistance. It is much better where possible to do it client-side with well-designed CSS (maybe, but not necessarily, making use of media queries and such).
There is an exception here too: if you want to send less data to the mobile client, instead of sending everything you would send to others and adjusting styles to alter the display for small screens and finger-based input, then you need to either try detect the UA's properties or given the user an explicit "full or minimal" option (which may put you back to offering an "m." URL or equivalent so a user can jump direct to the minimal version).
That's more convenient than having to change my user agent or simply having mobile versions unavailable unless I shrink my browser window down to phone-sized.