I realize that I'm in a less common industry, but I often end up using calculus and DiffEqs in my day-to-day work working with drones. To your point on system

I realize that I'm in a less common industry, but I often end up using calculus and DiffEqs in my day-to-day work working with drones. To your point on system dynamics, queuing theory pairs really well with calculus on that as well. Being able to somewhat predict how a system is going to behave under load based on measurements and an understanding of how the queues work is super super valuable for decision making.