simplest digital voltage selector circuit using common parts
No, your idea will not work, for two reasons, one which causes the problem of the other.

First, like most CMOS devices, it is prohibited to have any input to the 4051 chip more than a diode drop above the supply voltage.  So to use it to multiplex 12 volts, you'd have to run it on 12 volts.

Next, the threshold to reliably detect a "high" input scales with the supply voltage.   With a 10 volt supply, the you already need at least 7 volts to reliably input a logic high, and at twelve volts you will need more still.

The circuit you are objecting to is complex in the sense that it is built from discrete parts, but it is actually simpler than what is inside the IC.  More importantly, it is targeted to do what you need, without incurring the issues that make your idea of using a 4051 unworkable.