Hard to pirate without a source.
There is a lot of sources for Disney owned films, 4K, BD, DVD, VHS and film. If they never made any physical releases ever again, all of these sources become more attractive to collectors and their prices would rise. Lets say, 50 or 100 years from now they haven’t released anything… can you imagine how much would e.g. classic Disney cartoons cost on the secondary market? Probably hundreds if not thousands of dollars per movie, all which they would lose. Even today there have been DVDs that command hundreds of dollars on the secondary market because they have not seen a re-release since.
All true, but irrelevant to non-collectors (ie most people)
But not to Disney when they could get some of that money by relesing their properties. Also one could argue that there are others than collectors who would like to own Disney classics in the future as well, that is normal families.