This specific point about maintaining his own version of python2 to not move to python3 was addressed by Calibre contributors who did the job (that author didn't want to do) of migrating to python3.

I am very struck by this remark:

"Kovid has stated numerous times that any patches which work towards python3 compatibility without hurting python2 functionality or performance would be happily accepted. Oddly enough, no one has ever taken him up on that, though a number of people have insisted it is very important that he himself do that work."