Cellular carriers haven't had a stranglehold on phone manufacturers since the iPhone came out. I don't think carriers are demanding features from Apple.
As far as Android, you can buy plenty of decent Android phones unlocked and just choose your carrier - as long as you don't choose one of the CDMA carriers. Not being able to use Sprint is no big loss. That just leaves Verizon.
Even with Verizon if you have a phone which supports their LTE bands, you can use them on Verizon. I used my iPhone 6 and iPhone SE from AT&T on Verizon.
I thought it was the other way around. VZW's CDMA phones support GSM so that their customers can use them while traveling internationally, but you're unlikely to find a GSM phone that has CDMA just in case you want to switch to VZW.
There are plenty of cheaper CDMA phones that don't support GSM. Most of the higher end phones support both.
And then there is another scenario like I believe some of the Google Nexus tablets only support LTE on Verizon and won't work at all if there is no 4G on a CDMA network. It will work on 3G/3.5G GSM networks.
As far as Android, you can buy plenty of decent Android phones unlocked and just choose your carrier - as long as you don't choose one of the CDMA carriers. Not being able to use Sprint is no big loss. That just leaves Verizon.