Based on the earlier public released plans and interviews, that would be true on a market by market basis as the new T-Mobile converts B41 to 5G. What is in the latest plans is anyone's guess. Going to 5G would provide 100MHz wide carriers which would mean 5 times faster uploads before factoring in 5G speed improvements. Could one radio handle all of the available B41 bandwidth that Sprint has in numerous cities (up to 8 or 9 20MHz carriers)? Rumors are the Sprint Massive MIMO radios can only handle 6 carriers for a total of 120MHz, but I could be wrong. My understanding is also that T-Mobile small cells are all fiber fed which would free up bandwidth for phones. T-Mobile would likely go for different radios hopefully covering the full 194Mhz since they would only want 5G support.
In deals like the Shentel-nTelos and T-Mobile-Metro PCS the FCC has always had the firms either upgrade the firmware to support the new network or to give discounts for new phones. Your voice will only be heard if you tell the FCC. The FCC comment period is now open until early December.
If T-Mobile drops CDMA, then voice coverage will also be reduced given the vast range of 1x800. (I have observed it up to 50 miles away, but that is in rare cases). In the Shentel-nTelos merger, LTE coverage was mandated to extend fully over all nTelos 3G coverage. Once again this is something you should ask for.
Another area of concern is VoLTE support. I personally think Sprint will only offer it on phones it is currently selling. Hopefully T-Mobile will go back to phones such as the LG V20 and have VoLTE support. But always best to ask.
Looks like Sprint and T Mobile have been active around my house. I see at least 3 new small cells that have going up in the last 2 weeks and are active. I can pull a band 41 signal from 2 of them... the 3rd I'm assuming is Sprint or T-Mo? I'm not sure what the T-Mobile small cells look like. Its very good to see progress. Does anyone know if the small cells can broadcast more than one frequency? Ie... if the merger is approved and T-Mo turns band 41 into 5G... then I assume I will not be able to pull that in with my Samsung 9+?