Open alternatives exist; they have not replaced the proprietary alternatives. "Replaced" is the key word. AOL, Prodigy, and Compuserve are dead. There's still a company with the AOL name and I guess I can't quite guarantee there isn't some AOL-subscription-only service left somewhere, but they don't matter. AIM, ICQ, MSN messenger, Yahoo Instant Messenger, QQ, and Facebook chat all matter.
(Global penetration of the various services are very uneven; if you're an American you may think ICQ is dead but it's still big in Russia, for instance. Yes, I know ICQ and AIM are basically the same thing but the brand is still separate. Everything I mentioned is indeed still a going concern in at least one major region of the world,. You can of course add IRC, and depending on your mood you can add Google Talk; it may be XMPP and perfectly capable of XMPP federation but almost nobody realizes it, so in practice it's more isolated than you might think.)
I can't quite guarantee there isn't some AOL-subscription-only service left somewhere
AOL still has 5 million paid subscribers. Dead is hardly ever truly dead. Facebook in 2020 might still have 500 million users, but the game will be very different.
Poking at their marketing material, I'm pretty sure that's only as a dial-up service provider, and maybe some really limited media streaming features that are less subscriber-only rather than licensed by AOL as part of the package but not exclusive.
I was talking about the AOL that fit into the set of Prodigy, Compuserve, and AOL, which I tried to make clear but may not have made clear enough. AOL the online service with exclusive forums and custom client software and exclusive games and exclusive this that and the other with incidental access to the internet begrudgingly provided as a second-class citizen is dead, killed by the open internet. A company called AOL survived (with assorted drama), but it's just part of the Internet now.
