There are several costs, the biggest being the experience that is passed down from veterans. Like "best practices" for instance - especially niche, domain-specific knowledge, is not always written down, and thus, not learned by AI. Where I work, we have this 60 yr old literal neckbeard sysadmin in our team, and every now and then he drops an arcane tidbit, like why something behaves the way it does, because it was originally designed to meet some requirements in the early era of computing... or some such. Even though I've been at this for 15+ years, I still learn something new from him. And the other seniors in my team. And this is all leant organically, not necessarily because I has a specific question or goal in mind. AI can't replace that.

Same with IRL human interactions, which not only has a mental health benefit, it also brings you closer to the person, which in turn comes with many benefits - maybe you can ask a big favour of them, or maybe they might introduce you to some new band or music that you never heard of, or some gig that's coming up etc. AI can't replace such benefits that comes with IRL social connections.