For the portions in Arlington County, you’d have better luck trying to convince them to turn it into a busway.
The part that goes around the Arlington Cemetary is already a freeway with the exception of the at-grade intersection on Pershing Drive.
And since much of it has frontage roads, you can construct Texas U turns to convert some intersections to RIRO
I'm well aware of that. I've lived in Northern Virginia since 1974, and I remember well when I-66 didn't exist inside the Beltway even though I wasn't old enough to drive. My point was more a point about local politics than about anything else: Arlington County's government is dominated by people who will have no interest in converting an arterial road of that nature into more of a major highway than it already is and would be far more likely to be interested in downgrading it to less of a highway than it is now. The City of Alexandria is the same way, for what it's worth—Alexandria has little interest in widening certain roads to relieve congestion if the added lanes are for general traffic, but if it were a busway, that'd be a different story.
Of course, it's true that ultimately, because it's a US Highway, VDOT is in control, but they would be highly unlikely to disregard the local government's position. We've seen that elsewhere in the area—a few years ago, there was a proposal to designate the portion of Van Dorn Street in Fairfax County as a primary route (it's currently a secondary route), but the county government decided to oppose the proposal and it was dropped.
To some degree, I understand where Arlington and Alexandria are coming from—many residents there feel, and have elected people to reflect the feeling, that it's not their problem to pave over their neighborhoods to make it easier for people who live further out to drive through their neighborhoods en route to and from DC. I get it and I understand why they feel that way, although in my mind you have to balance that concern with the flip side that by resisting improvements you may cause negative impacts on your own quality of life due to traffic congestion, pollution, etc. I'm not sure that making US-50 more of a controlled-access highway than it already is would really result in much benefit in that respect, though. Maybe the segment in Fairfax County between the Beltway and Seven Corners could stand to be upgraded, but that would pose a lot of difficulties due to lack of space (especially between Jaguar Trail and Graham Road) and maintaining pedestrian access across the highway to bus stops and the like. There are a lot of people in that corridor, especially the segment I just noted, who rely on bus transit.
Arlington Blvd was built to very sub-standard conditions...truck weight restrictions (8 tons) were placed on Arlington in Sept 1937, shortly after it was constructed and US 50 moved to it.
In 1950 the restriction was defined from Fairfax Circle to VA 237 Ft Myer.
The 1960 Fairfax County freeway plan did have all of Arlington Blvd inside the beltway as a proposed new freeway. The 1974 version had it down to studying bus-only lanes and 5 interchanges (including 7 corners which was already in place by 1962). Two of these were built, on either side of the beltway.
A 1953 transportation study of NoVa did recommend turning Arlington Blvd into an expressway
I don't see how it could be converted and be open while that was happening, even if there is enough ROW (I have doubts) to have a freeway there with Texas-style frontage roads which would be necessary to access neighborhoods and commercial properties.
It already has grade separated interchanges in several places. And how did they keep 28 and 7 open as they converted them to freeways?
Far more space, among other reasons. Route 28 was a rural two-lane road when I was a kid. It's still far less dense than Arlington is, and it always will be (especially with Dulles Airport being on one side of the road for a good distance, thereby preventing development there).