I used to have a spotify subscription, but it felt weird to have nothing at the end of 6 months. I cancelled it and just started buying music regularly. I spend a lot less and I actually 'own' (ish) a decent sized library now.
I doubt that it would cannibalize much, because I really think that pretty much everyone who currently buys some of the content they already pay for in a subscription is doing so for a physical copy (which would not be part of the cherry-on-top trickle). It also would not threaten the subscription, because the library one could build from that would not reach "cord cutting" volume even after a lifetime of trickle licences.
As usual, it won't exist because content owners can never understand that they would not lose anything.
Or when i can not find the CD in Amazon i usually get from 7digital.