There's very little doubt that Riviera will sell out, whether it takes 4 years or six years or 10 years and whether or not it has restrictions for one main reason... it's at WDW! OTOH, the nicest DVC resort (Aulani) may never sell out simply because it's not near a theme park.
Until all rooms have been declared, Disney simply rents out the non-declared rooms for cash. Depending on how long that takes, from a financial perspective it's like selling the contracts multiple times (cash room rentals and then DVC purchase) on some parts of the property. So ultimately, there's probably little reason to be overly concerned with the fate of Riviera.
Disney is unable to rent all of the Riviera rooms, and the amount they take in is much less than the sales of contracts. In addition, when the rooms are unoccupied for DVC, the maintenance is still paid for by the member dues. On the other hand, when the rooms are unoccupied without being declared, Disney gets to foot the maintenance bill. By selling DVC, Disney makes money and they limit their risk. The DVC owners bear most of the risk. Disney's exposure is limited to the amount of rooms they have not sold and point contracts they hold. If a DVC resort is sold out guess what? Vacant rooms? Not Disney's problem. Damaged structures? Not Disney's problem. Wage increases? Not Disney's problem.
Aside from the increased exposure that Disney has, Disney also had a problem with capital being tied up. If the resort sells out in 10 years, that's money Disney can't be using for something else. Even if they took the cash and stuck it in mutual funds, they will get at least 8% YoY return.
I'm sure this was particularly evident for Disney during the Covid shutdown. If Disney makes much more money off of non-DVC resorts than DVC resorts, they would've been building much more of them. Instead, Disney has been creating DVC, including converting non-DVC rooms to DVC rooms (e.g., Wilderness Lodge, Poly, etc.). The latter doesn't actually add many rooms to the inventory.