I thought T-Mobile has been gaining more customers with their lower price and reasonable quality? And I have always assumed Sprint and T-Mobile merger will give T-Mobile more firing power to continue the fight.

I think people often view MNO in Cost / Data. In reality the Data is "free". The MNO will need a way to paid for everything from Spectrum, Telecom Equipment Lease, Rent, Fibre Connection, all of these are Fixed Cost. Unless something is different in the US I don't understand why the cost of Mobile plans are so much more expensive than European counterpart.