They had been doing ooh plunders the entire time they were being massacred down. They had the second dragon already funded. From this point there are four options.
1. Kill both dragons, overextending resources while letting them fund a third dragon with their economies still intact. This fails a basic risk vs reward metric.
2. Kill the first, instantly get the second, and let them begin to fund a third while their economies are still intact. See above.
3. Don't kill any dragons. This is foolish.
4. Let the first fly away, disable their economies in the meantime, and instantly kill the second. Economies now damaged, unable to send a third in a timely manner.
The correct decision was made.