I was recently on a trail that was mostly attentive with perhaps 5% surefooted - it was a double black diamond for MTB (it was a multi-use path). Two people in their mid to late 60s were comfortable hiking on it (I’d just tag it as surefooted since there’s enough spots you can’t really get anywhere interesting sticking to purely attentive). In this case having both a mtb_scale and foot_scale (along with any other tags) would make sense to me. 
Yeah paths with a very poor smoothness rating can still be simple on foot. Having a shin high rock in the middle of tha path that you have to walk around is still just foot_scale=attentive even though it’s smoothness=impassible (though it’d still be pretty simple to ride around on a bicycle). I think even for shorter urban paths having a foot_scale is useful as it tries to be a bit more holistic about the experience on foot.