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I don't think this will ever get implemented

I think it will. I saw too many altcoins to believe, that people are sane.

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The attacker would simply insert his data into UTXOs with an amount slightly above the dust limit

Then, it can be applied on all UTXOs older than N blocks, regardless of used amount. People will only go further, without taking any steps back.

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a set of BIPs for such "emergency mechanisms", which could be implemented rapidly in case to be necessary but would not be "normally" implemented

There are already some things like that. One example is signet: highly centralized network, but also highly stable. If decentralized mainchain will be spammy, then people may switch for more centralized solutions for a while, not because they are better, but because they won't have any other choice, if running a regular node will demand too much resources. There are already users, which cannot synchronize the chain in two weeks, and it takes them longer. It will only be worse over time, as it will take terabytes to store the chain, and as it will become filled with more complex transactions.

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This could of course also include things like an emergency mechanism for quantum computing threats.

We already have one optional path for that: https://bitcointalk.org/index.php?topic=5557305.0

Feel free to improve it, if you have any ideas. It is P2WSH, so you can use any Script you want, and combine it with your own requirements.