Obviously people vary, but manufacturers tend to target ~4mm exit pupil to fit most folks. Younger people have smaller pupils, older people have larger pupils, and all our pupils dilate in lowlight conditions. It’s why you see most 10’s have a 42mm objective or larger, 12’s having a 50mm objective or larger, and most people won’t go higher than 15x with a 56mm objective, etc.
I think 12x42 with a 1.4 magnifier will be fine for limited number of people in limited conditions. But if this is your primary means to glass, and that means in challenging lighting conditions and/or extended periods of time, I don’t think they’d be very popular.
I’ve moved away from a spotter, and I’ll only use lrf Binos as my primary. I’m in for the 10x42 with the single mag reticle. I might get the extenders for limited use, and if so, I’d just do the basic arithmetic if I really needed to mil something with the extenders on. At least for me, that would be a very rare circumstance.