On-chain privacy is not very hard. Pirate Chain (ARRR) has universal zk-Snarks, a technology that has been reliable for 14 years. Monero has universal ring signatures, a multi-decade-old idea invented by two of the guys whose initials are in RSA. These things are actually very easy to implement, and have been stably universally implemented on more useful blockchains, in the case of Monero, for 12 years now.
Lightning offers no real privacy, nor fungibility. Nobody is actually using Lightning's onion routing, so there's no fucking traffic on the mix network. A mix network with no participants might as well be a straight tube. You aren't making smoothie if you only dump orange into your blender. It is just pulpy orange juice. Your "privacy" with Lightning is a complete illusion. And it's already further impacted Bitcoin fungibility. Got a Binance Japan account? Try to make a deposit with LN. You can't, because Japan's regulators do not like Lightning! You've got no fucking privacy, your mix network is fake and gay just like your retarded protocol, you don't end up with fungible currency at the end.
On-chain privacy is the only way you get privacy. There is no privacy unless the privacy is universally implemented. With the ~$130M USD of real value dumped into Lightning, for virtually no fucking users on a protocol that wouldn't have scaled to accommodate any serious number of them anyways, we could have implemented on-chain privacy ten times over. Monero's upcoming FCMP++ upgrade is a thousand times more innovative and hard to implement than both zk-Snarks and ring signatures.
I am sick of Lightning shills repeating the same tired lies over and over again. None of what you say is true, except #1, and that's only because Core is lazy and decadent and does not care about making Bitcoin actually useful as money, which would require fungibility (and privacy is a side effect of that). They only care about justifying yet another grift of venture capital on yet another feature that no significant number of people will ever even want to use.