There have been proposals to stop the bounties from expiring. On meta.SE these have been received rather badly, but would be most likely welcome here, as offering rewards for unsolved problems has a great tradition in mathematics, so I'm proposing to have this feature. (In case you wonder, apparently the developers monitor the meta of each site for feature-requests, so if it has a positive feedback here, it has a chance to be implemented once.)
To summarize some discussion, the main counter-argument was that such questions would occupy the feature tab for too long. I see no reason at all why the bounties couldn't be kept while removing the questions from the featured tab (possibly creating an old-bounty tab).
User yo' said that standing bounties might cause people never awarding the bounties. While I don't think this would be an issue, it could also be countered in several ways. For example, after the first 7 days, you could be offered to pay 100 more reps to keep the bounty (in the old-bounty tab), and if you ever award the bounty, you would get the 100 reps back (or optionally you could also claim it back by removing the bounty).
If you don't support this feature, I would be very interested to know why and discuss it.