For me, I try to decide if players feel compeled to get the 'one right choice' when selecting options for their character. Currently the veteran players consider hex to be a mandatory spell and assume that it should be active at all times or the warlock is not doing the amount of damage they should be doing.
I agree and think the new 'Pack of the Blade' Warlock with its own minor 'hex' power kind of reinforces this notion. That being said, sacrificing your concentration to keep it up is, well, a sacrifice, and not something you would or
should do all the time.
The other single target level 1 warlock spell does 1d12 (if it hits) each round with concentration for up to a minute. Since both spells use concentration, you would never want to have both spells with the minimal ammount of spell slots that a warlock has. That means a warlock can take a spell that does 1d6 on every hit for the next 24 hours, or they can do 1d12 (if they don't miss) each round to one creature after each short rest.
This on the other hand...I just...don't. If your players think
Witch Bolt is at all worth it in any but the most contrived circumstances, I'm not sure 'veteran' is the term I'd use to describe them. You are almost always better off just using
Eldritch Blast, especially with Hex up, and especially with
Agonizing Blast. I don't think a poorly designed spell like
Witch Bolt is really a good argument for or against
Hex.
That's why I've gone with the 'concentrating on a spell holds a spell slot' rule at my table. That moves hex from being mandatory to just being a very good damage option. Any warlock who did not chose the 'one right option' would not feel gimped at my table.
I must say I don't really agree. Warlocks have enough limitations on their usefulness, particularly in the direct damage department (outside of EBing everything in sight). Limiting
Hex in this way does not do them any favors, and there are still times when you want to sacrifice concentration for other spells. Even with your ruling, I still wouldn't touch
Witch Bolt with a ten foot pole.
That said, your point about
Hex being, if I can paraphrase, 'an undocumented spell tax' is well taken, if arguable. I'm not sure this entirely correct though. I think one could take the position that it is possible to make a decent Warlock without
Hex. Personally, I feel even more strongly about this issue with the Ranger and
Hunter's Mark, and wish they had just made it a class ability rather than revert back to the tiresome 'favored enemy' shtick in the revised Ranger.