That's assuming perfect privacy from the original channel peer. You still have to have an on chain transaction locked up with a peer and all payments initially route through them. This gives a huge amount of metadata that can be combined with timing analysis for subsequent hops.
Yes payments are onion routed but if the entry node is static and there's no obfuscation packets or anything then timing attacks are trivial. Combine this with centralizing routing nodes as you touched on in another reply you have a scenario where you have good privacy from the recieved but terrible from state intelligence.
Most Bitcoiners are under the impression that institutional adoption is good and state intelligence is pro Bitcoin so they believe this threat model is reasonable, but time will tell.