Question about GSM and CDMA

So because Verizon is as CDMA carrier getting an unlocked phone for verizon is pointless because you can't use it on any other carriers since sprint is in a different band right? Getting an unlocked phone for ATT isn't as bad because you could also use it on T-Mobile and a few other small carriers. What if phone manufacturers made on version of a phone that had both CDMA and GSM radios in all bands so that they could operate on all carriers. If that was the phone that verizon sold only in a locked form, they couldn't really say you couldn't have it on their network could they?

I don't really know a lot about this stuff, so if I'm missing something please tell me.