If physics were no object why not have a better tank?

Walkers generally have lots of problems compared to conventional warfare. Mainly they aren’t resilient enough to stand up to much more than rifle or pistol caliber weapons without some hand waving. Joints are super vulnerable if you want them to still move and you can’t mount as big a gun as you could on a conventional vehicle on something that walks. We don’t currently have something that is portable and powerful enough for what would be needed for most sizes of mech. Also depending on how tall your mech is they are way bigger targets for equivalent firepower compared to a tank and by virtue of following a human body pattern are less armoured.

So yeah if physics is not a problem then have an anime mech powered by unobtanium with vibranium armour and a giga buster gun, they could fill every role in a conventional military. Mechs are all about the rule of cool. If you want realism then maybe something like an exoskeleton to help infantry carry more kit or bigger weapons. Up to a little bigger than a person they could potentially be useful in dense environments like urban or jungle where things like tanks are vulnerable to ambush by infantry.