I think it is quite a bit worse, especially when you are not looking at just raw dpr. Poison Spray does the same damage as WB, albeit with the significant disadvantages of 10' range and a Con save. Fire Bolt does about a point less damage but has a much better range and also, along with other attack cantrips, benefits from advantage. Then there's the big benefit of the cantrips in that they don't use a spell slot. You can spam one each round and still have your concentration on some other effect. You may miss some rounds sure, but you also don't have to commit to doing so each round if the situation changes, and you don't loose the spell if the target moves out of range or goes under cover. Sure, if your target is going to just stand there and take it round after round, WB will really do a number on them, eventually. But usually that only happens if a) they are heavily engaged with the rest of your party/allies such that they cannot approach you to force a concentration check and for some reason cannot back out of range or quickly move through some cover, or b) they are chained up/incapacitated and you are torturing them to death.
While I can see situation a coming up occasionally, I'm not sure that WB is the best use of the spell slot, depending on resources and options available, in the race to zero hp. It can be better to buff one of the fighters and pew, pew, pew with a cantrip, or to MM the target and pew, pew, pew, as more damage earlier in the fight can be more valuable than more damage spread out over many rounds. As for option b), by all means, just make sure you bust out cool lines like "And now you die young Skywalker!"
