You’d only want to use a mixer where your transaction represents a small fraction of the total volume (otherwise it doesn’t provide much anonymity). An exit scam isn’t so appealing in that case. If a mixer charges a 1% fee, with an average delay of 1 hour, the ongoing fee earns more than a one-time exit scam of all transactions in flight after just four or five days. It takes time to establish a reputable mixer and scale up, so your wouldn’t ruin it just to get under a week of profit early. Maybe they’d choose to do the exit scam if a really anomalous huge sum showed up, which is so large they can’t mix it effectively anyway. I don’t know if $4M is a lot by those standards.