I think this problem is far often overblown specifically because of how spellcasting is pretty limited in 5e D&D.
You've got a range, concentration, limited slots, limited actions, limited spells known/prepared, components and saving throws to worry about, not to mention the fact that there are some soft limitations like sight, recasting restrictions, penalties on miss, AoE, and enemy abilities.
Its pretty difficult to end an encounter with one spell unless that encounter was created without a good grasp on what makes fights challenging in the first place. Could the DMG do better to establish what that would look like? Yes. But one spell fights were usually doomed from the start even without a spellcaster.