Like the 495 HOT lanes, people will complain about them and then use them. People don't know what they want nor what they need when it comes to infrastructure.
Though in the case of I-66 inside the Beltway, they will not have a (congested) freeway choice as a "free" alternative. The "free" routes most likely to be used by shunpikers probably the same as today - U.S. 50 (Arlington Boulevard), U.S. 29 (Lee Highway) and the George Washington Memorial Parkway.
Also, is it safe to assume that there won't be any non-toll new highway construction (at least part of the new lanes being tolled) ever again? I would figure not.
That may be a reasonable assertion, though I am not very good about predicting the future. If someone had told me that Md. 200 (ICC) would be built and open to traffic in 2011 back in 2000 (when anti-all-highways Gov. Parris Nelson Glendening was still in office), I would have been pretty dismissive of such talk. But things happen, and the (toll-financed) road is there today.
Pretty soon Maryland and DC will look to improve and HOT 295 from the Beltway to the 11th Street Bridge, I bet.
The only interstate (note lower-case "i," as I include any highway improvement project between an adjoining jurisdiction and Maryland) HOV/Toll project I am aware of that has gotten on-the-record interest from anyone in Maryland is at the American Legion Bridge (yes, that's part of I-495). That
interest was expressed in the form of a joint letter to the secretaries at Maryland DOT and Virginia DOT and signed by the members of the Montgomery County Council and the Fairfax County Board of Supervisors.
Note that the counties would presumably not be involved in funding or operating HOV/Toll lanes at the American Legion Bridge or anywhere else, so the ball is effectively in the court of the state DOTs and the governors.