Once Blizzard shuts down the Warcraft III servers, the way to play the game at that point will be to buy a third party CD of the game and put it into a computer. I would estimate this might happen around 2025 or 2030.
Some years later – and this might be a while but I don’t know, so let’s put it at 2060 – there will be a time when Microsoft updates the Windows Operating System to run on some new kind of computer chip that is not backwards compatible with the Intel x86 instruction set. You might not know what that means, but essentially even if it takes 100 years at some point the world will stop making computers that would be capable of executing the code that is stored on a Warcraft III CD.
Some day, although I couldn’t say when, new computers will be blocked from running old programs. We already see this in the Macintosh world; as far as I know, no Macintosh computer for sale today is capable of playing the Warcraft III game in the form that the old “Mac Warcraft III CDs” were stored.
At that point, either:
- Blizzard makes a new version of Warcraft III that runs on whatever kind of computers are used in that future
- Blizzard does not bother
You seem to be interested in the worst case, so let’s assume the #2 possible future happens above. In that case, on some kind of future computer that can’t run present day programs, someone will probably make an emulator for Intel x86 programs because we have so many. At that point, you would be able to play Wintermaul on the emulator, but it will be really laggy. Also, keep in mind that all of this is only single player. Multiplayer would most likely die when Blizzard shuts down the servers in the 2030s, unless you are OK with pirate or hacky unofficial servers. The unofficial servers will most likely continue to exist, but some users might choose not to use them, for obvious reasons.
So, that is what I expect will happen “after WC3 inevitably fades to dust”, and even in the worst case, you can still work around the problem with an emulator.
As for your other question, “Can the custom games not be taken out of WC3 and offered independently?” the answer is no to you, but yes to Blizzard. Blizzard could take out the game and offer it independently, but per the License Agreement, you are probably prohibited from doing that.