I think the real issue with weigh ins the day before is merely that it makes PED use even more rampant. It’s no coincidence that fighters seen to have taken the most advantage in recent memory have in fact been caught using diuretics... If you had comprehensive testing then it wouldn’t have to be such an issue, could even be something fighters negotiate on etc. Some guys naturally can shift weight more than others, obviously more so young guys, but that’s just yet another reason boxing is generally a young mans sport. There can be huge drawbacks to doing so as well, so weighing in the day before, in itself, doesn’t really upset the playing field imo.