Moto -> Just had 95% of their US employees wiped out. Obviously the USA is not a deep priority for Lenovo going forward. Unlocked phones non-carrier phones outside of Apple have never done well in the US profitability wise.

Sony -> They've almost gave up on the US market. They were even considering shutting down the smartphone division altogether b/c it's so unprofitable. If Xperia X doesn't pan out, and I bet it doesn't, I don't see them spinning more cycles on B12. Their lack of updates shows the lack of care.

OnePlus 3/Nextbit -> Basically Oppo's R&D division hoping to break into the US someday. Nextbit is a startup money bleeder. They're both looking for markets so they can be profitable someday, and they'll take what they can get in the meantime.

ZTE -> already given up on the low end wrt to T-Mobile. It'll be interesting to see if they continue to try to support T-Mobile on the high end. At this point it's just throwing good money after bad.

Phones must be certified by T-Mobile to run on T-Mobile's networks, and this includes E911 certification. It is illegal under FCC auspices to do so otherwise, which is why they've asked OEMs to turn off B12, and most have been happy to do so.

It almost never makes sense for cheap phones to get updates on T-Mobile. A premium phone only makes sense if you can win market share from an already saturated market, because it doesn't make sense to roll a custom ROM for a tiny fraction of T-Mobile's base.

Sprint and Verizon have no real issues because there's significant CDMA deployment in China. AT&T's phones basically just need crapware added to their global GSM/LTE phones. It's the sheer unprofitability of trying to win market share in the T-Mobile B12 special snowflake that is going to narrow the field even further in the next two years.