Even better: never leave The lightning network and you’ll never really have these issues

Largely, no. While the full privacy implications of lightning network use are extremely complicated and still being researched (and the best practices updated with that information), it absolutely is not as simple as open/close channel, done.

what if the output of lightning came from mixed coins?

What if every coin is mixed, eventually? You think exchanges will just shutdown their bitcoin trades and throw away their likely biggest revenue stream?

Exchanges are already prejudiced against "unhosted" wallets. I think they'll eventually stop accepting deposits from any source other than other well known exchanges.

I've moved "unhosted" coins into at least 5 different exchanges at this point with zero issues.

What if a ransomware group starts to airdrop a few Satoshis to everyone?

those can be excluded easily

Output of a lightning channel is almost certainly more likely to contain mixed coins. The UTXO model is hopelessly outdated.

I strongly disagree, UTXO model will be necessary to scale smart contract platforms in the future as the account model will never hold up with the necessarily expensive sync operations it performs.

UTXO model, or a derivative will win out when it comes to distributed blockchains that can scale way beyond current distributed blockchains capacity.