I can do 100 ‘classic’ reps in 1 set but I don’t. 

Anything more than 30 pushups is a test of endurance more than anything else. Endurance is important if you need to work on that. For me I prioritize intensity. I’d rather spend valuable energy on challenging variations across multiple sets. I find 3 sets of 30 > 2 sets of 50 or 5 sets of 20.