Wen v5?
https://groups.google.com/g/bitcoindev/c/iVLHJ1HWhoU/m/ydjeTH6rAwAJI propose to fix this by removing the difficulty reset rule from testnet4 through a flag day hard fork on 2026-01-01.
Nothing changed since then. The
source code for proposed fork is still, where it was. Time will tell, if developers will decide to hard-fork into a new testnet5, or patch the old testnet4 with soft-forks, or do something else. And because their goal is to keep the price as close to zero as possible, I guess testnet5 will be worth even less than testnet4, so I don't know, if you would want to list networks, where each new version is less and less tradable. Because you didn't want to list signet, and obviously, nobody lists regtest, where six blocks are produced every second.
However, what happens when those ill-gotten dev coins are effectively frozen/stolen?
And what happens when keys for signet will be stolen? So far, it is running since 2020-09-01, and it has around 265k blocks. If keys for signet will be known by everyone, then it will turn into a network, where signet challenge would be just set to OP_TRUE. So, it would change signet into regular testnet, without 20 minute rule, and with regular difficulty adjustments. And this is also possible: a lot of signet coins are now owned by developers, so if any weakness in ECDSA will be discovered, then it will be very similar to proposed testnet5, where 10.5M coins are premined.
Also note, that the current signet uses 1-of-3 multisig. Which means, that for each public key, there is at least 80k signatures. If there is any weakness, related to address reuse, then it can be exploited on signet. And also, because signet miners sign blocks, filled with transactions, provided by users, then it is possible to do some attacks, by sending particular transaction IDs, to encourage a given signet node, to sign a weakened block header, which can be combined with other ECDSA attacks, and used to discover the private key behind it. Because a signature is just a relation between two public keys. If users can pick z-values, by sending particular transactions to the network, they can potentially alter R-value in a particular way, which could make it easier to attack.
Or have we outgrown that mentality and are prepared to let this puppy run as a hot potato game for ~10 years or when the block reward is pwned(?)
Anyone can release any test network, at any moment. It is just a matter of convincing the community to use your network for testing. In the past, developers had a problem with testnet3, so they made testnet4, to fix bugs. Now, they have two problems: one is buggy testnet3, and another is buggy testnet4. They can stop supporting old test networks in the future, but they cannot "turn them off", if there will still be some users, willing to maintain it. Then, old testnets will become just some altcoins, maintained outside of Bitcoin Core (or they will die, if nobody will want to fix bugs, and if old clients will be exploited by new attacks).