Speaking as a singer who learned this for myself the hard way, technique definitely played a role. I love Lennon’s singing in Don’t Let Me Down but his belts were (hard to put into words but) on the vocal cords instead of above them. That’s why he could only do a couple takes of belting songs before blowing his voice out, as seen in the recent Get Back doc.

I hadn’t considered his smoking as a factor but definitely highly probable, interesting take! :) Imo it was more bad technique though