Getting rid of the mobile lounges would have negative impact to the employees in the mobile lounge shop (not just drivers but also mechanics and others, less than 100 total) and that would be a union issue - but like the other issues you mention, not truly insurmountable.

The nice thing about the mobile lounges and plane-mates (there are actually two different types) is that the do provide benefits that buses simply cant provide (no risk of steps to climb/descend, barely any exposure to weather conditions, ability to get crew/pax to/from terminals to planes that are on hard-stands, ability to move many more people than buses). Replacing that capability with entirely new style of mobile lounges probably IS cost-prohibitive, not least because the airlines have to proportionally pay for those costs which will be passed on to PAX in the enplanement fee - making IAD less competitive after many years of successfully getting the enplanement fee down. Once the capital construction plans are completed the lounges can go away easily.