I suspect, and I have no knowledge of this, but I strongly suspect the idea was to ramp up DVC very quickly into a national network (remember, the initial plan was not to have WDW properties in abundance). Build out.If they just kept the term to 50 year contracts they would have minimized this issue.
But as mentioned the person making the decision to end the contracts all in 2042 must have had a reason.
That person probably is long gone but maybe they left the reasoning and plan for 2042 behind.
So all these resorts ending at the same time initially was probably a good idea - VB, HHI, OKW, as long as the never opened DVC Long Beach, DVC NYC Times Square, DVC project in the mountains (can’t remember if it was Utah or Colorado off the top of my head) all end in 2042. And, the “premium” WDW DVC Disney’s Boardwalk Villas all ending at that time makes sense.
As DVC expanded more in WDW, they continued with the strategy until it started becoming Lower than 40 years. As I recall there is a 40 year minimum in the Florida timeshare law, but maybe I am mistaken on that. At that point they reset to 50 with the first new resort (SSR), and have stuck with it since.