Unless you are currently very old or very ill I seriously doubt that even you yourself will be able to access your iTunes library by the time you're on your deathbed. The funny thing is that you probably won't care either.

As successful as iTunes and Apple currently are, media distribution is still undergoing massive disruption. It's likely that something completely new and utterly amazing compared to any current online media store will win out in the end. Distribution services that provide a sub-par experience on the majority of hardware out there are doomed to fail even sooner. i.e. iTunes does not provide a good user experience under anything but OSX, and even there it's love/hate. Amazon and Google Play are serious contenders despite the artificial iOS obstacles Apple is heaping in their paths.

One might argue that iTunes will live on even if other stores win out, but legacy support only goes so far. Once iTunes stops making Apple money they'll wait a respectful period of time and then quietly shut everything down.

I know many will disagree with me on iTunes' fate, but all I have to do is to live long enough and I'm virtually guaranteed to have the last laugh!