The voucher is good for one year from the date of issue.
This is a key element of why offering a voucher for "$9,950" probably doesn't end up costing the airline much more than a voucher for, say, $1,500. Very few passengers are likely to spend anywhere near that sum annually on DL (after all, the spend threshold for Platinum Medallion is $9,000) and if a passenger were to choose to spend the voucher on a couple of long-haul business seats, it's likely that those seats would have otherwise gone unsold.
A few instances they've offered a lot of money and still nobody takes it.
The attractiveness of "store credit" with a fairly close-in expiration date (one year) can be poor for infrequent travelers. Also, sometimes the substitute flight offered is incredibly inconvenient; i.e. the substitute for the overbooked ORD-SDF flight was nearly 24 hours later and wasting an extra day in Chicago isn't necessarily feasible for everyone.
Another benefit to the issue of vouchers is that it may get some infrequent flyers to spend more on your airline than they otherwise would.
For example, let's say an infrequent traveler takes a voucher for say $400 on airline X. Being an infrequent traveler, there is a decent chance that passenger may not have flown again with airline X in the next twelve months. But now that they have their voucher, they feel the need to "use it or lose it."
So they go to book a vacation flight somewhere, and they look at flight costs. Now psychologically, the person will feel inclined to want to book a flight that costs at least $400, because any flight cost less than that is leaving money on the table. And since it is very unlikely that the total cost is exactly $400 for the flight, they book a flight that costs more than $400.
So to conclude the example, they book a vacation flight that costs say $512. The passenger feels like they got a great deal, because they only paid $112 out of pocket for their second flight, AND airline X gets $112 in extra spending that the passenger would not have otherwise spent with them.