Pretty sure they can slowly repurpose LTE portion of M-MIMO to 5G nr so that shouldn't be a problem in the very long run. Just like Clear equipment, and those have served their purpose quite well over the last 6 years.
It is disappointing that Sprint does not have a not Massive MIMO 5G option. I assume the price would be significantly less for less robust solution that would allow for both 4G and 5NR. This would allow more places to have 5NR sooner.