What can happen? A license suspension, or the involvement of debt collection agencies?
There being no federal preemption of the matter, states have the power to regulate operation of motor vehicles on public highways, and the main manifestation of this is via licensing requirements. Indeed, §40 of the state constitution allows and regulates taxation of motor vehicles. RCW 46.16A.030(1-2) requires registration for all vehicles (not just commercial vehicles, which are defined in 46.16A.10 and are subject to different registration requirements):
Vehicles must be registered as required by this chapter and must display license plates or decals assigned by the department.
(2) It is unlawful for a person to operate any vehicle on a public highway of this state without having in full force and effect a current and proper vehicle registration and displaying license plates on the vehicle.
RCW 46.16A.030(5) further states that
Failure to renew an expired registration before operating a vehicle on the public highways of this state is a traffic infraction.
RCW 46.20.289 then says that the DOL
shall suspend all driving privileges of a person when the department receives notice from a court ... that the person has failed to respond
but that is limited to "moving violations":
a notice of traffic infraction for a moving violation, failed to appear at a requested hearing for a moving violation, violated a written promise to appear in court for a notice of infraction for a moving violation, or has failed to comply with the terms of a notice of traffic infraction, criminal complaint, or citation for a moving violation,
"moving violation" means any violation of vehicle laws listed in this section that is committed by the driver of a vehicle, while the vehicle is moving.
The offense is operating a vehicle (not possessing a vehicle), this driving with expired tabs is a moving violation. License suspensions have happened to at least 300,000 people, but I know of no statistics on how many one-time offenders of the tab requirement get their license suspended.
True, RCW 46.16A.030(5) states that failure to renew an expired registration before operating a vehicle on the public highways of this state is a traffic infraction. However, the definitions section, RCW 46.16A.010 defines who RCW 46.16A.030(5) applies to, and it only applies to operators using their vehicles for commercial purposes. ``Used for commercial purposes'' means the carriage of persons or property for any fare, fee, rate, charge or other consideration, or directly or indirectly in connection with any business, or other undertaking intended for profit[.]" 18 U.S.C. 31. The driver's license, registration, and even insurance requirements are based on the definition in RCW 46.16A.010. A vehicle not used for commercial activity is a consumer good and not the type of vehicle required to be registered and the “use tax” paid of which the tab is evidence of receipt of the tax.” Bank of Boston vs Jones, 4 UCC Rep. Serv. 1021, 236 A2d 484, UCC PP 9-109.14. There are a lot of other cases supporting what I've stated; look them up. Get a law dictionary and look up the terms in the code; they get you with the legalese. Driver or operator, for example, means someone who uses public roadways as a place of business and therefore must pay for that privilege, i.e. obtain a driver's registration and driver's license.