There are cards that were being retired and already sold out right as I got a yoto for my kids that we would have been interested in getting now or when they got older. I'm sure there is a decent amount of content we missed out on. Examples include Beverly Cleary series (ralph the mouse, henry huggins, risby, socks), splat the cat, matilda (english version).
I am under the assumption that the content is not deleted from the servers or else people who purchased to cards would not be able to hear the content they paid for. Why not let more people access the content that is already there through digital card only.