I may be thinking about it wrong but with a Dec use year, aside from the technicality of selling "2022 points" or selling "2023 points" to
DVC, it's really only a difference of 3 months.
With a March UY, you sell them back 2023 points (not "next year's") and you will have points available to book reservations starting March 1, 2024. You can use those to book a vacation for next summer.
With a December UY, you sell them back "2022 points" (technically "last year") and you will have points available to book reservations starting December 1, 2023, which is just 3 months earlier. You can also use those to book a vacation for next summer.
So I believe that, aside from the 3-month timing difference, the main difference would be that the Dec UY contract would indeed get an extra use year overall since everything expires in January 2064. So, one contract has 40 years of usage and the other 41. Everything else, including the 2023 prorated dues which you pay from the day you sign would be the same. I don't know that 40 years vs 41 years (2.5% difference in overall usage) should matter that much if March is more appealing than December.