[a-dub]

i think if one were looking for a more philosophical approach to calculus, apostol would be a good choice.

i personally have found calculus to be useful in a fair number of domains i find interesting. anywhere computers process signals from the physical world, it tends to show up, whether it be sound, vision, or other signal types entirely.

not to say that graph theory and abstract algebra aren't important (especially in things like discrete algorithms or cryptography) just rather if you want to build systems that interface with the physical world, you need the original language of physics as well as all the linear systems and sampling theory to go with it.