> Somehow both the MVNOs and the national carriers make money.

The history of MVNOs seems to indicate otherwise. Not a lot of them survive as independent entities; I think because there's not a lot of profit to be had, and most MVNOs are setup where the business is captive to a given operator. You'd need to reissue SIM cards to each customer in order to switch to a new operator, and that's expensive; not to mention some customers have operator locked phones that wouldn't be able to switch, so you've got to develop a plan for them as well.

SIMs can technically be remotely reprogrammed through SMS messages: https://www.theguardian.com/technology/2013/aug/01/sim-card-...

I don't think you can change the IMSI remotely, or at least, you're not supposed to be able to. Most US MVNOs use IMSIs assigned to the operating carrier, which makes it hard to switch the operating carrier. Some do use their own IMSI, which you could then conceivably change network agreements and use a different network, but with today's networks, your users might have phones band optimized for the old carrier, and not have good reception with the new one.

Wasn't the 6502 manufactured by Motorola who sold to all of those companies?

No. MOS, formed by ex-Motorola employees, developed the 6502. Commodore bought MOS in 1976.