Toll the Dead is pretty simple, since it just deals damage once. Most spells aren't that simple, though - they're going to involve creating status effects or summoning a thing or future interactions. All of those are things that need to be stress tested in combination with all of the other things in the game. So, there is a question at some point of whether adding a new spell is worth the effort, particularly if it is similar to something else that already exists. (I'm not saying that it is or isn't worth it in any given case).
Another consideration is decision fatigue. Having too many options when you are picking spells to learn (or cast) can be frustrating for some players. There is probably a sweet spot for how many spells should be available to provide enough variety but without making decisions overwhelming. There are ways to mitigate having too many options, but their effectiveness is going to vary from player to player.
