I use the neverssl.com trick and I worry: if browsers stop supporting plan HTTP, doesn't it mean this won't work anymore?
No, worst case you'll see a security warning. Or chrome will add an exception for such a site. This is a well known problem that I'm sure will be addressed. But with the current state of the web (~20% unencrypted) it's not really an issue yet.
If it were addressed in a way that would break captive portals forever, so much the better.