They've been one of the most consistent recipients of negative reviews that I've personally seen over the years, as far as retailers go. Not small issues either, sometimes pretty egregious stuff.
Personally I only tend to regularly check a small number of shops, but from what I see online there's been a LOT of retailers pop up within the last decade; a few closings too ofc, but quite a lot of new ones. I certainly don't have stats to hand by any stretch, but if I had to put a bet one way or the other, I feel like I'd say more sellers have opened than closed post-turn of the millennium. By quite a few potentially (if I were to be bold).
As for TM, not sure what other folks see, but my observation is that it seems to be the one brand of RIF that's always just consistently selling and selling out at shops. I'm not sure if there are any other brands (within the UK) that have businesses dedicated to them the way we have the likes of E6 (doing both sales, specific parts and tech work) but again, TM just seems to have the most of that type of thing 'orbiting' it by a long stretch; far as I can tell. People always want more NGRS and MWS guns/mags and the various pistols/mags. Which is a very good thing imho.