They're so big that it's worth blackhats spending significant resources to game their algorithm. That induced them to implement a spam filter which is now discarding the ham along with the spam.

Which means that smaller search engines that aren't being targeted by spammers are now giving better results. That is a major long-term problem for Google if they can't avoid throwing the baby out with the bathwater like this.

People only use Google because it has historically had the best results. They'll get some way on inertia now, but that doesn't last forever. They need to fix this or they're ultimately in trouble, and we could be heading for a landscape where being a search engine above a threshold size is a liability.