LogFAQs > #964953748

There's nothing wrong with destroying every Dark Hold, spells that affect entire multiverse have been done.
As for why Strange still had third eye.

They said the Dark Hold takes a toll on you to use.
It can corrupt you like it did Wanda, but we literally don't know what the 3rd eye symbolizes other than...something like a toll.

Presumably destroying it did not remove whatever psychological aspects or mind opening perhaps that it did.
After all, destroying the copy Wanda has, which was her ONLY source of Dark Hold magic at the time, did not remove her mental corruption.