And if you get 100s results instead of 0?

And what if it's not 90%, but 50%? It's definitely wrong for me, I most often use it to exclude irrelevant results, and I'd imagine it's also wrong in general case for such a targeted search engine

The "effort" is using the same typo detection algorithm you use for non-quoted search to display the same warning

Also the benefit of a warning is that you can press a correction link to automatically fix and restart search, which is better UI