No one is saying that mobile readers shouldn't ever share a URL on the "m." subdomain (despite them, quite literally, saying that). The criticism is that Wikipedia even has 2 separate resources for the same article and that the default behavior on "m." is to share the mobile resource while that's not the same case for the "en." subdomain (or other language variants). The only thing being said is that it should be consistent or it should be seamless - "en." always takes you to desktop and "m." always takes you to mobile or both should take you to the same page and the page should be responsive to the device you're using at the time. When users are saying "don't share the 'm.' page", they're only saying that because it doesn't exhibit the behavior of the "standard" page (which I'm only calling "standard" because that's how it was stated despite the fact that I don't agree that's a proper name for it).

I agree with criticism of Wikipedia’s behavior. I disagree that ordinary visitors to the website should need to or indeed should manually manipulate the URL of the page they’re on in order to share it with someone else. If you’re sharing the URL directly with a specific person who you know feels strongly about which URL they received, then sure, do it to be nice. But I disagree with prescribing that one URL should be the default (when it’s not even clear which one it should be!) and complaining about other people not respecting your preferred default URL.

> I disagree that ordinary visitors to the website should need to or indeed should manually manipulate the URL of the page they’re on in order to share it with someone else.

Everyone agrees with you on this point. You shouldn't need to manually manipulate anything but, because Wikipedia keeps a desktop resource and a mobile resource, there is a distinction and so you do need to manually manipulate it if you're sending a link from mobile to someone else and don't know their device. That doesn't change the fact that sending a mobile URL should always show the mobile page because that is, in fact, a unique resource and, therefore, has a unique URL.

> because Wikipedia keeps a desktop resource and a mobile resource, there is a distinction and so you do need to manually manipulate it if you're sending a link from mobile to someone else and don't know their device.

As I've explained, even knowing their device is not sufficient, because someone on a desktop device may prefer the "mobile" design (which is responsive) or someone on a mobile device may prefer the desktop design (because it apparently contains more features). And if you're sharing the link with multiple people or publicly, you obviously can't accommodate mutually exclusive preferences.

That wasn't my point, though. My point is that the mobile version and the desktop version are explicitly different resources. They do not load the same page. Therefore, they should have separate URLs. The very first parent comment stated that sending the "m." URL should load the mobile version of the page regardless of the device being used and they're correct. The fact that people are redirected there from the main URL is the problem, not the fact that they're sending the link to the mobile version since that's the "correct" behavior according to URL standards.