Small rule changes can invalidate an army. Even small points cost changes can make your list unusable. If your core units go up 10 points you're now 50 points over what you should be at. And maybe you need those core units to get your other units and it snowballs.
It's not about tournament play. Those guys will just buy a new army, speed paint it in a week and be done. It's usually more casual players who keep getting rug pulled when they build for January's rules and February's rules fuck them over.
You're right, it doesn't have anything to do with tournament play.
Do you really only ever build "an army" of exactly whatever the points cap is? That's weird, and boring. You build a 2000 point army for 40k and never intend to change anything? 1000 points for a star wars legion army? Even without rules updates, just simply other players in your area changing up what they're taking can "invalidate" your army making it effectively useless. Locals decide to start taking more vehicles to games than your "I'm at the maximum cap for 1 standard army" can handle and what, you don't want to get some anti tank units to swap in and deal with it?
If you have been playing whatever game for a significant amount of time and don't fall into the weird trap of deciding to never paint another model for an army again or whatever, then as I said you'll likely only need to acquire(buy, print, whatever) and swap out a couple of units. In this case because it's individual models and well under 20 total for Trench Crusade(and most skirmish games) that's effectively a non-issue. If you know the points updates are 2 or 4 times a year, oh no you'll have to paint 1 different guy. If you've been doing this long enough, you may even already have the stuff to swap out due to edition changes, balance updates, or simply swapping things around in your list so you aren't literally playing the same set of models for years at a time.
And specific to Trench Crusade, 99% of the paintjobs I ever see in youtube thumbnails, the occasional reddit post, the few times I've glanced through their reddit, look like shit anyway. It's a couple of base coats, then a wash or AK streaking grime or whatever other acrylic or oil wash, and maybe some splatters of blood. These people spend minutes on a single mini for an army of 10 guys with the latest slap sponge chop technique or whatever and it's horrendous.
edit: I just went to the TC reddit, and like I thought most of it is just base colors and covered in grime. There's a few clean metallic paint jobs, but then no shading on the metallics just the occasional silver highlight on gold that always makes it look like fake plated jewelry that's worn down. This is one of the rare decent paintjobs, especially for being an entire warband and not just one model
Most of the people there are following the grimdark = grimedark bit where everything just needs to be covered in filth at all times.
