Retro rubbish. There’s a very good reason why zips were invented and why they are so commonly used. Button flies are fiddly, so increasing the risk of making you look like you’re loitering at the urinal a bit too long, and hard to do up in cold weather (and there doesn’t seem to be much else in England). Unless they are of the rivet type in jeans they also are prone to falling off, rendering your trousers immodest and unusuable until you learn to sew or find someone willing to sew them back on.
There is another Room 101 within Room 101 which contains button flies on underpants. Underpants do not need fasteners of any kind unless you are some sort of weird masochist who likes to dice with danger every time you visit the loo. And if you are, why not just get zippered trousers and be really careless doing them up instead?