When will the version 3 die?
When users will stop using it (which may be never, if there is strong enough community to maintain it, as an altcoin).
And how will this happen?
By dropping the support for it, in the next versions of Bitcoin Core. Which means, that any new bugfixes and improvements simply won't happen on testnet3 anymore. It will be left as it is, without any changes. If some people will agree to become a new developers, then it may have a chance to live as an altcoin. But most likely it will die, because some people believe, that if something is not supported by Bitcoin Core, then it is not Bitcoin (which is not true, but if enough people believe in that, then the network will die quickly).
some users will not update to continue supporting version 3
Yes. And if they will encounter blockstorms, or other attacks, then they will have nobody to blame, because if you use outdated network, then you are responsible for keeping it alive, if you want to use it. The same would happen, if someone would decide to run Satoshi prenet version, with 20-bit difficulty blocks. Of course, you can use it, everything is decentralized, and so on, but if you will see, that your client blindly follows the longest chain (instead of the heaviest chain, with the most chainwork), then you can only blame yourself, because your node enforces those outdated rules (not to mention OP_TRUE OP_RETURN bug, Value Overflow Incident, and other issues).
