Even if David could afford to sue Goliath, and even if he won, Goliath would just lobby for small changes to the law to further strip rights away from all the falsely accused Davids.

I'm not saying the little guy shouldn't sue; people should stand up for themselves whenever they are accused of wrongdoing, and especially when they know for a fact they are innocent. The problem is, even when suing and winning sets a precedent, the big players just move the goalposts again. There's never going to be an easy solution that satisfies both content creators and content consumers short of massive copyright reform and hard criminal instead of soft civil penalties for making bogus DMCA claims.