but to strengthen the rules to reject transactions of a certain format requires a hard fork
Why? Rejecting transactions is not a problem in any soft-fork. Accepting more transactions than usual is a problem.
Imagine an extreme soft-fork, that would reject all transactions, and would allow only coinbase transactions. Peter Todd described it there, it is perfectly valid soft-fork:
https://petertodd.org/2016/forced-soft-forksWhich means, adding any kind of "expiration" to the transaction is not a problem at all. You have a node, you receive a transaction, you see "Oh, it's five o'clock! That's all for today!", and then you simply reject that transaction. And then, if most nodes do the same thing, then you cannot send transactions after 5 PM, because it is a soft-fork rule, and if some miner will include it anyway, then the soft-forked network will reject that block. And if soft-fork has 51%, then it becomes a consensus rule.
But yes, the hardest way to reject a transaction, is to include a double-spend. Then, all other transactions, spending the same coins, will be always rejected in the future, because of already existing network rules.
the latest one relates to how participants of p2tr do not know all the scripting conditions of all methods to spend. they just blindly sign a part not realising that part can be used against them
Yes, of course. If you sign a TapScript branch with any OP_SUCCESS, it can be always used to attack. The same will happen, if some soft-fork will not reach 51%. And yes, signing a blind TapScript branch is stupid, because it could be just "OP_TRUE" or even simply "<attackerKey> OP_CHECKSIG". In general, it is the first time, when OP_SUCCESS was introduced, and it can be painful to prove, that "there is no OP_SUCCESS".
but atleast now the flaws are starting to get noticed
Yes, it seems some things are getting more and more soft. And that process will continue. It started with soft-forks, instead of hard-forks, and soon we will reach another stage: no-forks, instead of soft-forks. People will start building more and more stuff, without going through soft-fork signalling, and all of that. The most recent case was Ordinals, that could be somewhat-safe if deployed as a soft-fork, but was deployed as a completely unsafe no-fork instead.
best suggestion .. start afresh and use a different payment system model.. make a subnetwork that actually meets its 6yo promises
Well, you can always start with unidirectional payment channels, and just disable routing. Then, if Alice has 1 BTC, and Bob has nothing, Alice can give more and more coins into Bob, but she cannot take any coins from him. And then, everything is simple: if you sent some signed transaction, then you passed some coins. It is irreversible. Bob will always get the last transaction, because it gives him the most coins. That model meets all assumptions, but has one drawback: it requires more on-chain transactions than LN, so it will not be deployed. But the funny thing is that
some Bitcoin ATM operators supported that proposal more than LN.
i might actually respect core devs again if they made a subnetwork that is truly functional without having to mess with bitcoins rules just to fit
You can deploy payment channels, that I described above, even without Segwit. The only thing you need is to eliminate malleability. And you don't even need multisig, because Paillier Homomorphic Encryption can do the trick even on P2PK:
https://duo.com/labs/tech-notes/2p-ecdsa-explained