Not being able to track does not absolve them of KYC.
You're right and they'll still do the same KYC which is already more than most banks do. And your bank example is correct so all coinbase has to do to be in compliance at the same or greater level of the banks is track ONE HOP of the transactions coming into or going out of their site.
Do banks need to follow dollar bills as they go to the third or fourth person down the line? Can they do that? Nope. And that's what coinbase is doing already with bitcoin transactions (and it's excessive and not reasonable, and they just need a good reason why they need to STOP doing it. Monero provides that reason).
And I think optics for tax case are terrible - IRS will say to court "Look, they just made it even easier to hide assets!"
Coinbase is doing nothing of the sort, it's just how Monero works and they can't help it. ADDING MONERO to their services is simply keeping up with the competition and being part of the industry. Their major competitor KRAKEN already supports Monero so Coinbase is just doing it too, to just keep up as every company must.
But, yes, sure you're right and nobody knows what will happen. We'll see soon enough.
In any case coinbase has said very clearly they're gonna add more currencies and given Charlie's tweets about Dash I think we can pretty safely say it won't be them, LOL
Maybe coinbase will surprise everyone and add Dogecoin LOL