I am not young. And I have played Warhammer40k for most of my life, so I am used to the Imperial system. It's just an annoying thing that is easily averted by a minor UI change. Besides, making this a "boomer vs millenial" (I am neither) problem is not really a way to discuss. Such minor things can have a bigger impact than you would think. Remember, most gamers are still 30 or below, the vast majority of strategy players comes from Europe, of which again the vast majority does not use the Imperial system. But it is the first thing they will see ingame. Quite a few will find a way to the Options menue to change that, but then again a good portion of the English Wikipedia entries on various warships are still Imperial system exclusive. So there you have a clickfest (that for me is already much easier as I have a multi monitor setup) just to get some historical more or less accurate ships...and a bunch of annoyed customers that go play something else. I have quite a few friends in the gaming industry, UI accessibility is probably one of the most important issues they have atm. Not only for younger people btw, but an increasing amount of elders or gamers that get older and have problems with their eyes, mental "speed" etc.
TL;DR: Making UI more accessible = profit, no matter how you think about it.