I'd allow it. Witch bolt is weak, and can use whatever buffs you can give it.
The sad part is that there is no ranged lightning cantrip, so your sorcerer won't be getting their Cha mod to damage with their default ranged attack spell. That is a glaring oversight that I hope is fixed the next time we get spells.
Not really. Unless battlefields in your DM's game are particularly uncluttered, Witch bolt's achilles heel is too great. The spell ending as soon as the target gains total cover means most times any other damage spell would have been better. And the spell certainly makes it look like it is a good idea to do exactly that! If you had a bolt of lighting bouncing between you and a spell caster, it would be stupid NOT to try getting behind cover.So, is this enough to keep Witch Bolt?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.