I don't care if my lightning channels, mixing partners, etc.. as re drug dealers and hackers.

There are virtually zero issues anymore with exchanges taking on 'dirty' Bitcoin.

As adoption increases, as flows and velocity of tax increase, it will matter even less 

However, even so, the incredible vast majority of Bitcoin is held by non-underground/illegal users. So I'm not sure what you are asking or your point is?

There is no benefit to 99% of bitcoiners, as most keep it with an exchange, custodian, etc .

But for self custodians, how does it help saving in Bitcoin, doing an atomic swap or other to monero, spending, and getting any change or other back via a swap.(Or all if a sale not a bit).

The chance of the swap Bitcoin coming from monero swap not only doesn't decrease the 'dirty' coin, it also is likely much higher to be 'dirty'.

So again, I don't get the purpose 

But to each their own.