Mixing whether it be done by centralized exchanges or by large anonymity sets increase the threat of domino cascade
Exchanges are just an example of a commerce transaction. You buy alpaca socks. The alpaca socks guy uses your coins and some coins from other socks buyers to buy wool, and then maybe the wool seller buys socks too. Lots of mixing going on there. If they are in-person transactions (or an online service instead of a delivered good) there is likely no trail of identity, or at best an incomplete one. Humpty Dumpty is not getting put back together, transparent or non-transaparent chain.
Blacklisting entire anonymity sets is legally and politically plausible
It is largely useless, since you are blacklisting coins that might well have already been spent. You also can't practically blacklist the downstream, as you can with Bitcoin (in fact some have argued some version of this is required by the law) because you would be blacklisting many and even most coins after some rounds of mixing.
There is a very narrow window of opportunity to actually know whether coins are unspent, before they are used by
anyone in a mix. And once they are used, it is only a short time from there before exponential spreading means they are then mixed all over the place and downstream blacklisting is impractical.
However, if the original "Schrodenger coins" can be blacklisted, then perhaps this is arguably a good result. Law enforcement can (with proper due process, at least in theory) at least try to blacklist the original criminal's coins without affecting people who may receive (perhaps retroactively) blacklisted coins in trade.
EDIT: Also, there is still no credible basis for a private key attack due to either de-anonymizing (traceability) or double spending. It hasn't been be disproven (indeed most of practical crypto is strictly speaking unproven), but continuing to repeat it as pure "there might be a flaw" is just uncertainty and doubt with no analysis backing it up and is not credible.
Why are you bringing that up? I hadn't mentioned that in our recent exchange and when I did mention it, I said the same as what you just wrote above.
You did mention it.
because with a crack on private keys only the attacker can double-spend his coins
Or did you mean "without?"
That being the case, what you said is untrue. Anyone can double spend, simply by spending on whatever fork does not survive. You buy alpaca socks on the doomed fork, the socks are sent to you, and then the fork is abandoned. You have the socks and the coins. If you are honest you can certainly send the coins to the alpaca sock seller, but nothing forces that. But who knows, if you are on the fork long enough, maybe the coins you sent were also double spent to you and disappear as well. There is no solution to this that makes everyone whole. Transaction finality is a judgement call.