Ownership of the content of the drawings would depend on the contract. Normally, 'work for hire' belongs to the person doing the hiring, not the laborer producing the drawings. One would have to work with a lawyer to craft a contract that did something different and allowed the drafter to haave any 'ownership' of the design or drawings.
example. Billy Bob Jones freelances evenings to draw up houseplans. If hired by a contractor to develop the custom house plans for Mrs Robinson, the contractor has frull ownershio of the plans and design. If Mrs Robinson hires our hero Billy, she owns the drawings and design. If Billy want to resell his work, he needs their authorization to do so.
Now, if he generates an original design on spec, and sells that design to either Gieseppe Builder or Mrs Robinson, he can resell those plans to anyone else (unless the agreement to prior purchaser included something on the order of exclusivity).
The question of long-term viability and readability of any file format, be it DWG, DGN, or DocX, is always questionable. If that's a real concern, then one should restict their archiving to non-proprietary formats, or at least documented ones. For Acad, that would mean using a non-Autodesk application that conforms to the OpenDWG spec. Autodesk has never gone out of their way to enable interoperablity between _any_ vendor. (And I include the DGN Import 'feature' in that assessment. the Brothers Bentley got snookered....)