I think the parent is referring to the possibility that one day, browsers might put up a blanket security warning for HTTP URLs instead of following redirects. Or they might try to be a little too clever with HTTPS upgrades.

In particular, the automatic upgrade feature in modern browsers is based on several heuristics rather than explicit configuration like HSTS, so there's always a bit of room for breakage there. For example, they don't even check if the server returns a 301 redirect, which can be problematic if the server wants to redirect to HTTPS on a different host/port or make some changes to the path.