Duckduckgo has seen some pretty strong growth [0]. Given that this is merely an option (most people won't change from the default, or will pick Google out of familiarity), I'd be interested to see how much of an effect this will have on their numbers.
I think a secondary issue Duckduckgo and other competitors have to contend with is the idea that nothing other than Google is worth the time of day. I know every thread on HN has people queuing up to say how DDG results aren't as good as Google results (or to say how they switched a while ago and DDG is good enough for their needs), but from a user perspective we can see a much more level playing field in Google vs DDG et al (2020) as opposed to Google vs Altavista et al (1998).