My response, not part of my original post, is that clients do not always do what is necessary to protect users and often have code bugs.
Yes and there is _nothing_ that can be done about that generally. Even the example you gave of address checksums are enforced purely in the client, and never show up in the Bitcoin protocol-- and no one is arguing that the txout size should be increased by 20% to accommodate them. You've already had something like seven developers respond (now eight, with me) to you telling you that these kinds of checks can only sanely be implemented in clients (and already are implemented, by me in fact, in Bitcoin Core).
If the client software is broken (or worse, malicious) all bets are off-- no amount of consensus rules can make them safe; by adding arbitrary restrictions you might cover up one or two corner cases in incompetent clients, but at a cost of handicapping Bitcoin and making node implementations more complex and buggy; leaving us with strange economically significant paramters hard coded into the protocol... and would likely fail to prevent the broken clients from actually losing any money. It's not a meaningful protection; and mishandling fees has never been a bug in any widely distributed wallet software that I'm aware of...
but it should not have been hidden from them by a list moderator.
They're not hidden, they're moved elsewhere in public-- according to the rules of the list, almost certainly by someone who isn't a Bitcoin Core developer-- presumably because you were simply repeating yourself in a non-productive way. As a reader of the list I'm thankful for that service. Just because a message is accepted doesn't mean that it's reasonable for a discussion to go on forever.
and somehow someone hacked a retail terminal to charge 300% fee
If someone is hacking your terminal you likely have much greater things to worry about than them making you pay higher fees-- like them directing all payments to themselves. Software is not magic. Sprinkling around an endless series of handcuffs to close off implausible what creates a spiked trap of complexity that would undermine the survivability of the system and provide negligible to no protection. You're welcome to disregard concerns like that add such things to your own software; you're not welcome to waste unbounded amounts of other peoples time demanding they do it for you in their own.