Let's say 999 out of 1000 times some signal penalizes spammy sites enough to get them off the first pages of search results.
Those 1 out of 1000 good sites that were penalized do mean that "Google isn't doing its job" in that its job is never complete, but that doesn't mean that it wasn't the right choice to make for its users (while it then tries to refine new signals to get a better set of results).
The crap filtered out can vastly outweigh the good results (temporarily?) lost, depending on the good sites that were filtered out. Yes, it sucks, but false positives always come with this type of filtering.
Also, remember that "best" is itself just a confluence of signals, and that the guy next to you may very well disagree with you for any particular "best" choice.