Even in the days before crypto, scammers could hide money in other ways. They need to catch him.
No. But he will have effectively given up all his known wallets. If he touches them, he’ll have lit himself up.
Would a "grey" crypto market refuse to trade (some) of the coins into another less traceable crypto?
No, but every wallet it touched would be flagged by AML software, making it ineligible for e.g. sale to an institution or deposit at Coinbase. That reduces its value, albeit not to zero.
What if he also spread 100btc among random active wallets of coinbase users, would coinbase ban thousands of users? take their "tainted" crypto?
If they truly are interested in finding him it's straightforward and within their power, just a bit tedious.
As for exchanges on TOR: maybe? I haven't heard of any and IRC was rough and expensive enough. Exchanging bitcoin on TOR sounds like a great way to loose all of it.
But sure, if he touches those wallets now, he will possibly leave a paper trail. Let's hope, for the victims' sake, that he does.