So what would this mean for web development agencies and the likes, who've made hundreds of thousands of websites (most without SSL) and would now be forced to convert them all to use https?

How many clients would be willing to pay to cover some of the time wasted doing that?

And how about the potential SEO impact? I mean, Google says switching to https shouldn't affect things, but I've seen tons of sites lose rankings after a change.

Okay, using https is a good thing and all, but the inconvenience caused by having to make this change could be massive.