Remember that with LTE, there's no longer a divide between the GSM camp and the CDMA camp; it's the same LTE, although each carrier operates on various bands.
With the merger, and as phone firmwares are updated and incompatible handsets are cycled out, the combined LTE network is compelling. With Boost's current target market of urban youth, the fact most current Boost phones only support the Sprint's CDMA2000 3G and not T-Mobile's GSM-flavored 3G is less of an unfortunate fit than with a more general MVNO.
[1] https://newtmobile.com/wp-content/uploads/2019/05/FCC-Filing... [2] https://web.archive.org/web/*/https://newtmobile.com/wp-cont...