So a mathematical anomaly with spellcaster buffs was able to do once what spellcasters in general can do regularly and that's good design?
I also said in that same sentence that was only one example of several. That one was memorable because it was late in the campaign and against a powerful foe.
If a wizard is winning every encounter for the party it's a problem with how the encounters are built by the DM.
Until I learned to build the encounter properly, I had the opposite problem with spellcasters in 3E. They would spend the first 2-3-4 rounds of combat de-buffing bad guys while the fighters, paladin and barbarian were wreaking havoc. I had to put the 3E equivalent of minions on the table as targets ust so the spellcasters wouldn't feel totally useless.