The problem with slides is that longevity depends very much on processing and storage. The storage you can deal with yourself (though "archival" storage can get expensive!). Processing is another matter. I have slides from my father and grandfather dating back to the late 50s (ie. nearly 50 years old). Some have lasted well, others have faded. There is no real pattern as to which brands last and which don't, so I think some of the variation can only be explained by less than optimum processing.
The exception is, of course, Kodachrome, which seems to last forever, and part of this is due to the fact that only Kodak can process this stuff, and quality control is more rigid than your average local chemist!
So with slides - pay for quality processing (or do it yourself), and store the slides properly. But now the technology is affordable, scan and store the images digitally as well. This acheives 2 things - it gives you an additional record of the image which you can duplicate as required, plus, once scanned, you need never handle the slide again for prints copies etc.
Colin K. Work, Pixstel