Let's encrypt wouldn't be nearly as widely used if browsers didn't require https for a lot of new features people want to use, though. And Google will lower the ranking of non-https sites soon, if they haven't already.
Having Let's encrypt available as a free option lets the browser makers force web services to use https, since "anyone can add it, it's free! (Never mind the increased complexity and even more things that can fail and break things in weird ways)".
I suppose it'd be harder to do something similar with email.
