"I bet one day people are going to make GPL'd custom clients for popular websites."
People do, it's just hard to do so against any website that has an incentive to prevent you from creating a third-party custom clients, which is every site that serves advertising, which is... uhh... basically the internet.
If people can build and maintain huge ad blocking databases, I don't see why custom clients and APIs for sites would be out of reach. For example, youtube-dl has handlers for specific sites, gets constantly updated to deal with changes and has a generic video scraper it falls back on.
I posted about this idea on reddit and someone showed me this:
People do, it's just hard to do so against any website that has an incentive to prevent you from creating a third-party custom clients, which is every site that serves advertising, which is... uhh... basically the internet.
But, see Hacker News: https://play.google.com/store/apps/collection/cluster?clp=gg... (At least, I hope that link sends you to a search on "Hacker News" on the Google Play Store. If not, well... search for "Hacker News".)