In reality, of course, your D&D world would be RIFE with anti-magic and counter-magic magic. Magic resistance and magic immunities.
A lot of it coming from wizards. They want protection from other wizards. They don't want the other wizards protected against them, but that's not their choice. Also, they want to protect their servants sent about tasks not quite important enough for the wizard, but important enough to want done. And there would be a certain amount of status lost from having your servants blasted. . . .
On the more basic level, counter-magic and anti-magic would be among the most useful things a wizard could sell, especially if it stacked.
Permanent objects would work better than spells, even for the master wizards. Who wants to have his tower laid open to every light-fingered rogue in the realm merely because you came down with the flu?
And that's not getting into clerics. Will the goddess of agriculture stand for the harvest being laid waste by a wizard duel? The god of sovereignty with letting wizards toy with kings? Just because they are in the same magic power league as the wizards doesn't mean they have to support a magocracy. Indeed, it's most likely to be that they can't.
