Scheduling in a larger org and/or with multiple equally busy people is a non-trivial, complex task; it makes sense to dedicate resources to the task. Good Executive Assistants are generally fairly smart folks, in my experience.
When the scale is substantially more and involves objects as well it evolves into multi-million $ ERM (Enterprise Resource Management) systems.
Trying to do more is a losing game, and AI assistants just paper over that. We all have finite time and attention. I think a pragmatic engineering approach is the right one here: consider that as a non-negotiable constraint, a fact of the physical world, not something to magic away.
Do I want the AI Agent to take my bank account and automatically pay some bill every month in full? What if you go a little over that month due to an emergency expense you weren't prepared for? And it's not a matter of "I don't have enough in my bank account for this one time charge", but it's "I don't have enough in my bank account for this charge and 3 others coming at the end of the month." type deal.
Agents aren't going to be very good at that. "Hey I paid $3,000 on your credit card in order to prevent you from incurring interest. Interest is really bad to carry on a credit card and you should minimize that as much as possible." Me: "Yeah but I needed that money for rent this month." Agent: "Oh, yeah! I should have taken that into account! It looks like we can't reverse the charge for the payment."
Yeah, no fucking thank you LOL.
Also this supposed use case is called "Autopay" and requires zero AI. A lot of people still don't use it. Even when it includes a discount!