Clea (my stepdaughter) was at art college some decades ago, and when she left home she left her artwork behind. Like so much, it was stored in the loft. The loft needs clearing, and she's been going through the portfolios, sifting and sorting and letting go -
Compare this process with what we need to do with material that is less emotional. The receipts etc that we need for tax returns need to be kept for a certain span of years, is it seven? But this limitation doesn't apply to our artwork, whether preliminary or finished - and so much can pile up, even in a year or a few intense months. After some years, some of the work is ready for discarding - it simply won't be taken forward, and if you need to look back on it, looking at a photo will do as well.
Yet it seems "such a waste" to discard these products of our creativity, of our searching for ... something we may not yet have found ...
Like Clea, I have many loose drawings from classes and courses in my portfolios; most are quite recent. I'm not ready to look at them yet, never mind get rid of them. As for work in sketchbooks - this is not a problem at all - it's tidily tucked away in books, is easy to access, and gets looked at relatively frequently; also I love to know the books are there, waiting, and that I can revisit inspirations and possibilities.
Loose work, like tax receipts, can be discarded bit by bit ... gather what's past its keep-till date and now not neededor wanted: throw it on the fire, and enjoy the brief flame. To have a sketchbook burn, though - that's alarming.