I've played mages and sorcerors at various levels. Generally, casters become less effective in combat as the levels go up due to the high saves and resistances of the opponents. You need those save or dies simply because the 1/20 odds of success beat throwing 20 fireballs. Fighters, on the otherhand, are this constant litany of pain inflicted upon opponents.
SoD spells are not useful in all cases; being ineffective vs constructs, undead, and certain other creatures depending on the spell. Excluding disintigrate, SoDs don't knock down walls or clear forests. A mage isn't going to have more than a few of those in his repetoire and a sorceror who's learned them has done so at a cost.
IMC we haven't used SoDs simply because of setting-specific circumstances. Wait, not true, I did try Finger of Death on the fighter but he never realized he was being targeted by SoD and the party casters were blinded by fog. IMO charm/dominates cast on the fighter are worse than SoDs because they move the fighter to the BBEGs side of the table.
I don't fear SoDs as a DM; opponents are just as susceptible to instant kills from the sniper/archer's criticals. Instead I plan on SoDs. Smart casters work defensively, throwing up lots of chaff via minions or summons and using their high mobility to best advantage. I'm fond of having Imp Invis casters flying high above the range of See Invis and cast spells with no visual component. To be really fun, I toss out a few Silenced tanglefoot bags or just a few ghost sounds to obscure verbal components.
Consider using polymorph magic to swap with one of the minions; the lich-goblin falls readily but the goblin-lich keeps casting from the secret passageways.
Yes, that requires the BBEG to be prepared. If the party catches my BBEG by surprise, they deserve to stomp him quickly. They outsmarted me and get their just rewards. If they don't catch me by surprise then why oh why would he not be prepared? I run big bad evil guys, not big dumb evil guys.