Fine in the sense of very good quality seems to be an absolute adjective, and since absolute adjectives are not gradable, so I am wondering if this principle applies to fine in the sense ‘of very good quality’ as well?
Regarding why fine seems an absolute adjective to me, let us take a moment to consider the following etymology of fine from the Oxford Dictionary of English and Wiktionary:
from old French fin ‘end’, cognate with Eng. finish
This etymology brings me the impression that it means ‘too good to be improved further’ or ‘so good as cannot be better’, so I feel like fine is much like an ‘absolute adjective’ such as superior.