Valid, but I'm still more concerned about "why should I spend a spell slot to deal 1d12 damage, when a Cantrip is 2d10".
Hmm, I actually used Witch Bolt quite a lot in my early playthroughs, until I realised that upcasting applies to initial damage only. I think it is a decent option early on, but without scaling of guaranteed damage I don't see myself using it past level 4.
It's an odd spell to commit to using every turn, even at low levels.
Who are you going to use it against? If you're fighting against something dangerous, you're better off casting Magic Missile or Chromatic Orb every turn for a significant increase in damage. On the other hand, weak enemies die before this spell can get enough hits in for the 10 turn auto-hit to pay off. I think the auto-hit is what makes it suck, because there is no point in committing your action to weak guaranteed damage for 10 turns against a single target.
I think it should be changed into one of the following:
- only 1+2 turns duration, but more damage
- one hit but jump to additional targets, i.e. mini Chain Lightning
- line AoE against Dex Saving Throw i.e. mini Lightning Bolt
- reactivation is a Bonus Action but spell ends if you don't use it
