I still have the leather band from the original S0 and it holds perfectly well and is very adjustable. I’ve even worn it on runs and it holds just fine (though the leather looks pretty beat up due to constant sweat/water).
Fair enough, practical experience would trump theory here. Good to know that magnets are holding up though, I keep a milanese loop as well and I have always been hesitant to use it during running or biking. However, I also saw in a few threads here people using the milanese straps and said works perfectly fine (which goes with what you said), just gotta keep it clean when the grime builds up.
Nice thing about leather is I have always liked that beat up (aged) look and it sounds like it lasts. I may have to invest in one now.