All major browsers implement HSTS preloading, and getting added is quite simple. A very large percentage of your average internet user's traffic is covered by this.

Preloading is a problem waiting to happen. It works fine when only a small portion of the internet uses it. But when you have 2 GB preload file with a few billion entries things are not going to work so well.

The idea is to make HTTPS the default before that happens. In the meantime, you can fit a lot of domains into bloom filter-like data structures.