That if you don't pay the license can be revoked at any time is probably indeed a myth. On the other hand, there is a chance big enough to worry about that many FOSS license could in fact be revoked by the copyright owners for other reasons, or if not revoked have distribution greatly hindered.

There are two issues (at least).

The first is that some such licenses may be "bare licenses". A bare license is a license to use someone else's property that is not coupled with an equitable interest in that property. Bare licenses can be revoked at will unless they state they are irrevocable.

Of course, you could have a bare license, and separate from that a contract with the copyright owner in which the copyright owner agrees not to revoke the bare license, so a license being a bare license is not necessarily a big deal except when it stands alone. I think that such an associated contract is reasonably likely to be found in most cases.

The second, and more serious issue is where does your license come from when you use FOSS software? This, as far as I know, is largely unexplored in the courts.

Let's say I've released Foo under a FOSS license. The license says it is irrevocable. You get a copy of Foo from me. You've got a license to Foo, from me, and it is yours forever.

Now you give a copy of Foo to Bob. Where does Bob's license come from?

There are two possibilities. (1) It comes from me. Whenever you, or anyone else, no matter how many steps removed from me distributes Foo the recipient automatically receives a license from me. (2) The Foo license allows sublicensing, and you use your sublicensing right to create a license for Bob.

The big question is what happens if it some point I announce that I am no longer issuing new licenses.

The sublicensing case is the most clear. Everyone who already has Foo retains they license (remember, I made the Foo license irrevocable). Their licenses allow sublicensing, so they can continue to distribute, giving their distributees a license (which includes the sublicensing right).

Unfortunately only a couple major FOSS licenses, I believe, allow sublicensing (Apache and MIT). Most of the rest either are silent on it or explicitly say it is not allowed (GPL).

So lets assume non-sublicensable Foo...Bob gets a copy of Foo from you after I have purportedly stopped issuing licenses. I sue Bob for copyright infringement. What is Bob's defense?

Note that I am not attempting to revoke the irrevocable Foo license with anyone--I'm just trying to stop create new* Foo licenses with anyone else.

Bob can't really argue that you gave him a Foo license, because the Foo license does not allow sublicensing.

I think Bob is going to have to argue that there is a contract between me and you, and part of that contract is that I will issue new Foo licenses to anyone you give the software to, and by not issuing Bob a license I'm in breach of my contract with you, and that Bob is a third party beneficiary of my contract with you, and has standing to counter sue to enforce my contract with you to issue him a license. But third party beneficiaries only have standing when they are an intended beneficiary, not when they are incidental beneficiaries.

It would be a big, very unclear, mess.