Seconding what tye said, the space usage is virtually nothing, and the attention old nodes draw tends to fall of faster than the utility drop-off. (Which is why it is good for people to use search tools to find existing answers - that makes the utility decay at a slower rate.)

But if you are truly concerned that some of your old messages have become a waste of space, rather than asking for time, energy, and money to be wasted developing a feature allowing your material to be expired, just go donate at the Offering Plate. Give whatever you think is reasonable to cover your estimate for the space you used, cost of backups, administration costs for handling your account, and bandwidth that people used in getting your stuff.

Multiply that by some fudge factor to take into account the fact that you probably missed something, and your next 5 neighbours have not covered their costs.

And then you can stop feeling guilty about the space.