But there's no reason to think that this trend won't continue, meaning the unused funds will just get bigger and bigger. That's funds taken from the reward pool at the last HF. Those funds need to be disbursed, so as to add value to the community commensurate with the value taken away at the last HF.
By the way, is there a way to see the amount of unused funds in the SPS?
There are currently 143,032.368 SBD in the pot. So there are now over 1430 SBD per day available, of which only 310 SBD are being used daily (and it keeps growing, because 10% of the rewards pool automatically goes into the fund).
You can also see this on https://steemworld.org/proposals
The balance is in the account @steem.dao but only a maximum of 1% can be paid out per day.
The budget is a maximum, not a guarantee. If stakeholders don't think that available proposals add more value than their costs, stakeholders are correct in not funding them. There is no guarantee there will always be more worthwhile proposals than budget. One option (which was discussed when SPS was created) is a burn proposal, but I don't think we are there yet; the funds accumulated are still reasonable and could be used for future worthy proposals, should they be made.