Just clicked on the "Try the new UI" option in A360, after getting frustrated with the current options (not seeing how to publicly share an entire project). The new UI pops up, giving a dialog asking to review the Terms & Conditions then either decline or accept it:

Declining the dialog has no effect. Instead of switching back to the "Old UI", the page reloads in the new UI and presents the same dialog. Endless loop style. There's no way to get back to the old UI, and there's no way to proceed if you don't agree for some reason.
Side note - Anyone remember those snarky "trick dialog" joke executables from the early 2000's, that would poke fun at MS with imaginary dialogs like this? This is like one of those, but actually real. 😉