Fanaelialae
Legend
a) I'm having difficulty recalling a time at my table that a caster cast the wrong spell and caused a TPK. YMMVThe thing people forget about wizards is it's very easy for them to mess up. Cast the wrong spells and they can be completely ineffectual. If all you can do is hit things with a sword you are not going to get the blame when your bad choice leads to a TPK.
b) A fighter can absolutely cause a TPK by engaging the wrong creature, thereby allowing the wrong enemies to engage the party backline and permitting the squishies to get squashed (or even just having to waste resources protecting themselves, rather than bringing a quick conclusion to the fight).