heretothere-
“I remember being taught in yeshivah that once something is a obligation it does not cease to be an obligation even if the original reason for it no longer exists.”
Usually you are correct. However what SJS is saying doesn’t contradict that. It could be that chazal never said that the woman must wash her husbands feet, they just said she has to do whatever is normal for a woman to do for her husband. The reason they said she has to wash his feet is simply a translation of that rule into the language of their day and age. In other words, washing the husband’s feet is the application, not the law.
I’ll give you an example. For issur v’heter, the gemara sometimes says you need 1 eid or a chazakah etc. But most poskim will tell you that a video camera or something like that is good enough, because the gemara never meant to say davka those things, they were just the application of the rule for those days, but the real rule is that you need to know, and a video camera suffices for that.
Similarly here, one can make the case that the rule was never davka to wash the feet, but rather to do what is normal in any given society. Therefore although the application might change, the rule stays the same.