very different target audiences. a TV like that wil be fine for mutltimedia, movies and probably a lot of consoles, but for desktop PC use it doesn't cut it. 40" is probably too big for a start for most users, it will lack colour reliability and will not really offer very good performance for main desktop-type use. Also, as mentioned the lag tends to be very high and so they arn't really sorted for fast desktop gaming. they don't offer refresh rates >60Hz either so motion clarity is not as good as a decent high refresh rate desktop monitor. there's no dynamic refresh rate techs like G-sync/Freesync either.
You really only have two valid points there.
1. 40" might be too big - there may be decent 4k 32" TVs out there worth looking at?
2. No freesync or Gsync.
The rest i would dispute. The colour accuracy on the TVs can be better than a lot of monitors, the lag on this one isnt too bad and certainly no worse than quite a few top end £1000 monitors I could name. We are only just going to get >60Hz 4k screens later this year so thats not a fair comparison as they dont exist in the monitor world either. Quality seems better than PC monitor manufacturers
But you are missing the big point. If you took this £400 TV, stripped out the extra cost tv tuners and stuff which would then reduce the lag and added freesync or gsync, you wouldnt need an extra £600 which is what PC monitor makers want for us which then ends up needing 4 screens before you find a decent one whereas the 4k TVs seem to have much less issue with backlight bleed than monitors do.