You wouldn't need more parking spots. For people commuting from outside the downtown core, Metro is already car-oriented, and operates 60,000 parking spots at stations (not including lots run privately or by VA/MD). For people traveling inside the downtown core, they could just take the bus--as lower income folks in D.C. already have to do because Metro doesn't go where they live.
The operating subsidy for rail is slightly less than for the bus, but that's dwarfed by the difference in annual capital expenditures (about $1 billion for rail versus $200 million for bus). Metro's fare recovery is a bit misleading, because most of its "capital budget" is actually for maintenance and repair, which is an operating cost.
Also, the bus serves much lower income people than rail--I'm okay with it receiving subsidies.
According to the FY 2019 budget [1], operating expenses of busses are much more heavily subsidized than rail (recovery ratio of 23% for the former if you look at page 35).
