What really hurts is when you donate something to a not-for-profit museum, and then they sell it.
True. Sometimes they have to deaccession items to fund other purchases.
One benefit with getting slides (or other items) into the hands of museums, is that information and pictures can be made available in digital archives. Photography departments of museums often has staff that will work all the pertinent metadata, do color corrections and make files look good, and then either make the files available for people to use for free, or against a fee, depending on the copyright situation and who actually owns those rights.
That's another benefit to society that is probably not thought about enough, and benefits those who do research on various topics; people that are enthusiasts, curators, or otherwise benefiting from the effort.
If the pictures themselves are interesting enough (subjective, I know), of course.
Just so that it's blatantly obvious - I have no qualms at all with the OP selling these items. I'm glad he found a buyer that has an interest in them. That is much better than them collecting dust.