Loosing? Learn the English language, people!

Sorry, I missed this.

Can’t go into all the reasons the above is incorrect

No, please do.

but no: em is lowercase.

Yes, as I said. See above.

In brief:
The word em is the name of a measurement based on, but no longer dictated by, the letter M.

Indeed.

Context and rarity clarify its meaning. It doesn’t need a capital, like it doesn’t need putting in bold or underlining. It is. We don’t need to gild the lily. English dropped capitalisation of all Nouns in Sentences just because they’re Names a couple of Centuries ago.

I'm not sure I understand in what way this is meant to be an argument. Plenty of names and specific nouns are capitalised in English. This is often to clarify different usages of the same word, e.g. 'the Bible' or 'the bible of the XYZ movement'. Part of the misunderstanding here seems to be the assumption on your part that just because something doesn't 'need' something, it shouldn't have it? I don't think I can really work out what you meant to mean, though.

There’s an argument for preference, sure! Why would I care about personal usage? I wouldn’t dream of arguing the point if you hadn’t ‘corrected’ me with such authority in the first place.

Where did I correct you? I said: 'It should always be ...' and that continues to be my view. A correction would be: 'It is always ...' or just the correct word/spelling/whatever.

Capitalising ‘em’ makes as much sense as capitalising ‘alpha’ every time you use it (eg: he was no Alpha male). Just because it once referred to a capital, doesn't mean it needs a capital. If it was about the shape, it could be argued for. Take the L.

It's actually a good case in point and very similar. I would here likewise capitalise 'Alpha male' and consider that better than lowercase.

Again, can’t believe I have such strong opinions on usage of em. Need to go to Nerd Nite, be among my tribe.

So, did you go? :)