I don't understand your description of the problem, it doesn't match how the old login page works at all.
1. The username and password fields have not been on the same page for like, 15 years. You havne't been able to get from one field to the other with just tab for ever.
2. If you mean the number of tabs needed to go from the username field to the "next" button, on the old login page it's still three tab presses
3. The obvious way to get from the username entry page to the password page is to just press enter while focus is on the username field. Doesn't that work with the new login page?
I must admit I have not really studied the differences. Not sure whether I could get the old login page back to do so. Probably not permanently, so not worth my time.
The new page just felt much heavier and slower, so I looked what I need to do to the stupid next button. For my UI I want: If it's not broken, don't touch it. Marketing and graphical designers go elsewhere. And there was nothing broken for me with the old UI. (Those with screen readers might have more substantial insights in one direction or the opposite, I don't know)
1. The username and password fields have not been on the same page for like, 15 years. You havne't been able to get from one field to the other with just tab for ever.
2. If you mean the number of tabs needed to go from the username field to the "next" button, on the old login page it's still three tab presses
3. The obvious way to get from the username entry page to the password page is to just press enter while focus is on the username field. Doesn't that work with the new login page?