The last points you will get are those from 2041 and none can be used for a reservation that ends any time after Jan 31, 2042, e.g., those with a Dec use year will get Dec 2041 points which will be unusable for post Jan-2042 reservations, and that includes using them at any other DVC resort that is not expiring in 2042. As to borrowing points, Disney could "temporarily" suspend any right to borrow during the last two to three years of the resort. It could also suspend banking beginning a few years before 2042 to help prevent too many points accumulating for reservations in the last two years of the DVC period.
I seriously doubt DVC will do anything that would extend the use of any points or ownership interests for any purpose beyond Jan 2042. That would be a legal mess -- does it create an expectation of an extension of the existing term at the same points per night as already exist; what modifications would be needed to the existing POS and membership agreements to allow such; does doing so impact on its ability to start selling the resort as a new resort with a new 50-year term starting Feb 1, 2042; I expect that DVC's lawyers would strongly recommend against any idea of allowing any usage of the expiring points after Jan 2042.
My guess is that DVD will probably want to do a major refurb/redo to the resorts at the time and start another 50 year DVC contract that, of course, will be similar to Riviera, where future resale purchasers will not be able to reserve other resorts. Also the total points applicable to the new 50-year resort and thus also the points needed per night for any room will likely be greatly increased from what they are now. Those members that still exist as owners in the last few years of the resort may be given the opportunity to purchase a new contract before the expiration that will begin after expiration at a "discount" from the new regular per point price from DVD that will likely be more than three times what the per point price is now, e.g., BWV was $75 a point in Jan 2002 and now 20 years later is $230 (and that $230 buys only 20 years compared to 40 that was bought in 2002), and, of course, the actual cost increase for needed points will be higher because the points needed for any room per night will be higher.