We shouldn't need backwards compatibility built into the OS when we have perfectly functional compatibility shims that can be isolated from the rest of the system or run with reduced/user privileges. VMs, DOSBox, Wine, etc.
Okay, fair. Instead of perfectly, call it good enough most of the time.
People with some specific legacy need can run an old OS on a computer isolated from the rest of the network, but there's no reason to have such security-compromising backwards comparability built into every OS install by default.
Also worth considering that the big companies with mission critical systems on abandonware would certainly "lobby" to orgs like Microsoft to keep them from having to dust the cobwebs (whatever that looks like).
