If T-Mobile moves quickly, it could cover 80% of the U.S. with 5G at hundreds of megabits by the end of 2021. It will probably require several months to re-organize after the merger and the major impact may be in 2021. T-Mobile may pause but won't stop.
The money is budgeted; to get approval to buy Sprint, T-Mobile agreed to maintain capex at the level of the two companies combined, about $10 billion/year. That's enough to upgrade 20,000 sites a year and cover more than half the country extremely quickly. Verizon and AT&T would almost have to respond. (This is over-simplified but I believe correct.)
Sprint's 2.5 GHz spectrum is golden because it has excellent reach. Most of the country can be covered without adding new towers. The 64 small antennas of Massive MIMO mean that 2.5 GHz has reach comparable to 1800 MHz and possibly better.
The towers are available. Adding Sprint gives T-Mobile more towers than it can put to use. Originally, T-Mobile intended to keep 10,000 and retire 35,000. Many of the latter are on 5-year contracts.
The contractors have the capacity to meet that target. They were startled when T-Mo stopped building in December after the companies had staffed up. They'd love the work. Ericsson & probably Nokia are ready to ship the equipment. Sprint brings Samsung to the table. The equipment is available.