I'm the maintainer of Select2, and I'm interested in working together to fix the current lack-of-text-input issue.
I removed support for hidden inputs (<input type="hidden">) because there was no non-js fallback. Non-js users would be presented with absolutely nothing, and most likely any validation would be failing (pretty silently) because of that. All in all, it wouldn't be a great user experience, and it was something I wanted to avoid. Now, I didn't realize that people were using a text input as the fallback until about a week ago, and because of that much of the code is expecting a <select>, which is great because we don't have to special case that much.
The problem with the old implementation largely revolved around initSelection, a badly named function that handled the job of converting the value attribute to a set of data objects. I'm interested in working together to figure out what would need to be done to have Select2 function as an autocomplete for Drupal, but my knowledge of how the Drupal internals work is very limited.
We are tracking this issue at https://github.com/select2/select2/issues/3022 as well.