Who wears the cost though? It'd also cause a lot of frustration at the mechanics on both parties. Given that it would be mandatory, I'd dare say you'd see an increase in pointless "you need this for a rwc" items.
Quite frankly, I work 6-7 days a week, i don't want my day off spent waiting around at the mechanics for them to go over it. There's also a lot to say for those "questionably legal" modifications, if you get caught out - i.e something comes up (death, birth, etc) mid modification so you don't have time to tidy up loose ends, or just lose track of when reg is due.
Is body rust a thing they check for in a roadworthy? I thought it was just leaking fluids? On that note, I was dribbling fluids for the last few months - granted I pulled my car off the road when I was leaking fuel, that shit's scary, but a slooow drip of coolant? And my car would be pulled off for that despite it being mid-repair.
Some people just don't care about their car - I'd like to say they shouldn't be driving it, but I used to be one of those people - I genuinly couldn't spare a weekend to service the car, and I didn't really care to be totally honest, didn't mean I was less entitled to drive a car because it was old. Granted mine was always atleast cleanish, panels werent falling off etc... but I doubt it'd pass a roadworthy. Shifter boot was torn, so It definitely wouldnt have, actually...