Once upon a time some 'clever' people [the BBC for one] though it a clever idea that once they'd broadcast a Television Series they could re-use the media for their next recording/s and taped over the old.
Sadly this was at a time BEFORE end-users had the facilities [at home] to record what was broadcast....eg. was before the advent of VHS/Beta in the home.
Lots of series were thus lost permanently [unless you star-jump to Ursa Minor and catch the signals from decades ago...
Examples were proggies like Doctor Who, where entire series/episode sets were 'lost'. Then one day some arcjhivist was rummaging through the ABC vaults [in Australia] and found some old copies that were used to broadcast here...and many were thus recovered.
One of my personal faves...Callan is still missing virtually all of the first 2 series [in B/W]...and are almost certain to be gone for good.
I remember watching them...in the late 60s ....but they'll be 'no more'.
That's just plain frustratingly annoying....and shows what a lack of fore-sight can really do.
Meanwhile....VHS/Beta tapes do not last 'forever', either....so dubbing them to DVD [with or without remastering] is vital...and DVD/CD media is really an unknown quantity with regards to viable life-span.
Anyone creating [or distributing] art/video/music etc needs to be pro-active with regards to issues of Preservation.
Not everything we do/have is or deserves to be treated like the disposable society we seem to embrace so well.