It's a collective action problem. How do you incentivize an upgrade while allowing for seamless fallback? For billions of devices made by thousands of manufacturers? Which access hundreds of different carrier networks around the world?
I think you need some industry leader to push for this. Apple could do it, or possibly Google although they have a hard time exerting influence over Android device manufacturers.
Well, here's the deal. They can't do anything as it would be a potential quality of service issue to the end user if GSM were disabled currently. Can't blame the vendors either, they built the hardware years ago that operators continue to operate. It's like wanting to blame Ford and Chevy because you bought a car from the 1950's and it doesn't have airbags.
T-Mobile and AT&T are the national carriers supporting GSM through most of their footprint, AT&T is phasing out their network at the end of the year. T-Mobile moved to A5/3 to try to slow down eavesdropping a bit. There are more rural carriers who aren't in a rush to upgrade and probably still use the older ciphers.
Currently, I am on T-Mobile. There are a few areas that never saw HSPA deployed, and voLTE is not supported on all devices and also suffers separate outages. GSM is needed in those situations as fallback.
Operators would prefer to not have customers complain about outages due to accidentally having GSM turned off. So many phones don't have the option to disable.... though I'd argue now with low band LTE, the footprint is becoming larger than GSM.
I would expect Apple to whitelist 'disable 2G' option sometime in the future for AT&T sims. If not just disabling it outright (unless gov pressure gets in the way)
EDIT: as noted below, some manufacturers/OS you can set 3G only. Android is pokey about it on some devices. With Blackberry 10, I have the option to change network generations on T-Mobile, but not if I put an AT&T sim in.
I think you need some industry leader to push for this. Apple could do it, or possibly Google although they have a hard time exerting influence over Android device manufacturers.