I'm not so sure about what you say about large mixing. https://bitmixer.io/faqs.html claims to have 2000 btc available, around $500,000. I argued that exchanges function as a mixer of sorts, and they're going to have a lot more btc lying around.
The comparison of Tor to BTC was mainly comparing mixers to relays. Just like you need every relay to store logs if you're going to go after all of them to break the chain, the same holds true by mixers.
2000BTC isn't very much at all, especially since the mixer having 2000BTC doesn't mean they can securely mix 2000BTC.
Exchanges will most definitely be storing logs, so that won't help much.
Comparing mixers to relays isn't valid either, with BTC the attacker will have access to the blockchain, which is basically the worst possible attack scenario for Tor.
And in any case, there isn't enough BTC transaction volume to reliably hide large amounts of BTC.
In the end the process of money laundering ends up being significantly more complicated than with "real money".
Don't go and compare Tor to BTC, just don't. They share nothing from a technical PoV.