The main reason to buy park tickets would be to reserve park days. At this point, there's no rush, as the parks have plenty of availability. Also, it's not clear if reservations will still be required that far out; we might be back to regular FP by then. The other reason would be a fear of price increase, but I don't see WDW increasing prices in the near future. I could be wrong, but I don't think there's currently enough demand for that, especially with offerings still reduced and half the resorts closed.
Once purchased, your tickets retain their purchase value. The purchase price of a wholly unused ticket can be applied to the purchase of a new ticket. If your new tickets are more expensive, you pay the difference. If your new tickets are less expensive, Disney keeps the extra.
From my perspective, it doesn't make sense to tie up money with a non-refundable purchase. I don't see a benefit to immediate purchase, and if your trip is postponed, Disney has hundreds or thousands of your dollars you can't get back. Will you eventually use the credit? Probably, but it does lock you in, and there could be a better use for that money between now and 20 months from now.