mmWave's best use is in crowded stadiums/concert venues, and other outdoor crowded areas etc. Not certain when we will get back to that with Covid-19 so active here therefore mmWave equipment has a bad ROI right now.
I keep my phones for a long time so I would want all layers of the cake plus Vo5G. Personally I need to stay on Sprint since my T-Mobile network phone lives on b71 in my urban home inside the beltway. Fine with what I have right now, but without nearby Sprint site conversions, I would move to AT&T if my current phone breaks (or CDMA is dropped) and I was forced to be only on the current T-Mobile network. (I get up to 100Mbps on Sprint at home.)