is there a way to see the amount of unused funds in the SPS?

The balance is in the account @steem.dao but only a maximum of 1% can be paid out per day.

But there's no reason to think that this trend won't continue, meaning the unused funds will just get bigger and bigger.

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.