If the world is given no choice but to give you money, you're not going to be careful with that money. DHH argues that careful engineers would use microservices a lot less.
I also don't think he's arguing for carefulness at all - he's saying that if it wasn't for the free money, engineers would be forced to move faster and break more things to make quick cash. Understandable from business investor perspective, but has nothing to do with careful engineering. He's basically saying "you're so careful I am not making my money, fuck all that and deliver already".
Ensuring implementation details don't get mixed up is indeed nice in some ways, but cuts both ways: it makes upgrading core libraries harder in case of security issues and such.
Another time an unrelated change of another team created a security issue because they improperly used a singleton class from our team's code, and our code became vulnerable because of that. That one was caught during security approval, fortunately - we'd be fucked if it went from CI straight to prod, or if they tested only the changed parts.
It's not just about CI. It's about integration testing, user acceptance testing, security approval, etc. If your system goes from CI directly to production then yeah, maybe you don't need microservices that much - but there are still benefits.