The problem with "learned everything ourselves" is that you might have niche interests and you miss things. Things I learned that probably I wouldn't have by done by myself: computer architecture (memory, buses, cpu-s, instruction set), and related VHDL/Verilog; how complex is synchronization (implementing from scratch synchronization libraries); different programming paradigms (functional languages); compilers & operating systems (kernel modules, etc.); various types of maths (dsp); algorithm complexity analysis.

Some I ended up using more during my career than others, but knowing more definitely reduced my tendency to think "ah, that should be easy".