To reference Yabapmatt's reply below. If the SPS is returned to the DAO, it can certainly do whatever the community wants with the funds after that. If you want these funds, or any DAO funds, put to a use then a proposal can be made to do just that - be it distribute them to current holders, used to fund some other project, or be burnt.

I think it's a fair argument that the unclaimed SPS could be considered burned, but I feel it is better that it simply be returned to the DAO and then a separate determination can be made of what the community wants done with it.