Of email users? Or or thunderbird users?

I don't think looking for a majority of email users is worthwhile, as the majority of email users are not going to use anything other than the webmail from their provider, and maybe the one built the in to their device.

As for thunderbird users, I'm not so sure. It seems those who feel they'd use thunderbird if the UI was different in this thread still seem to be unsatisfied by these changes, while those who like the UI the way it is are asking for the new UI to be optional

This is a fair question, and you're probably right. On second thought, the average user will stick to webmail, anyway.

Any data to back this up? (Yes, I also have only one data point).

Citation needed.