The point is not power but hardware compatibility. Emulation only goes so far and many, if not most, weird esoteric hardware systems from the 90s depended on idiosyncracies and strange usage of standard busses and weird interactions of the CPU. Emulation almost always breaks this.
also most of those USB adapters likely won't support true hardware switch interrupts, Direct Memory Access, or raw bus control to talk to other cards, which almost every special ISA card actually needs at least one of these to function.