Unfortunately it is very difficult to keep both login paths working forever. Login/signup are critical paths for users, and there are many variations available. There are local logins, oAuth2 logins (via Google, Apple, Facebook, Github, etc.) and SSO logins via DiscourseConnect. Plus, instances can customize various parts of the process (login required or not, choice on what fields to show, etc.). Maintaining all this for 2 completely different UX flows is very, very hard long-term, it’s prone to breakage, and we can’t run that risk on such a critical path for users.
Happy to hear more feedback about this in particular. Probably best to post it in the announcement topic: Introducing our new fullscreen signup and login pages
I should add that on mobile and tablet, the full page layout has some definite advantages. It’s easier to navigate, especially on mobile phones, the software keyboard behaves much more nicely.