Guys like Symmonds could never go faster than 47 in an open 400 and guys like Rudisha would struggle at 1500m.
Based on what? Rudisha does some serious mileage. I think someone said he trains with Kipchoge. Rudisha in peak 800m shape could probably run around 3:30 and 13:45-14:10. He is just a better, speedier version of Sebastian Coe who was a 45/46, 3:29, 13:30 guy. If Rudisha actually trained for distance, then he would be even faster at the distance events (and slower at the sprinting).