As this prove without doubt, it can be done this way, this starts seeming to me more as a way to improve sales of new OS, PC, MB and CPU wich will get the Bios update. (and do no forget new RAM).
Finally the damned marketing is the game always.
Not entirely sure about what you are trying to say.
Can Microsoft have the OS load the microcode? Of course. Will they? possibly. They probably already do, unless they found a different way to get the same / better result.
But the problem is... What do you do with what ever loads before the OS? Unless the microcode in in the UEFI, (BIOS) you do not have any boot time protection.
As i've said before. It's quite unlikely that anyone actually will target home users with any of this in a effective way, but a patch should definitely be provided by the motherboard providers. The argument is made that it would cost to much, but in reality... Nope. Just updating the microcode isn't a complicated process, and putting it in the download section with the drivers for your hardware, with a "BETA" disclaimer, would be sufficient.
Hell. People update the microcode in BIOS.rom's just so they can run unsupported Xeon's in consumer boards. It's not that hard. But on a user level we generally have to remove something else to add the new code, with the proper library's that the manufacturer has, this could be omitted.