Your premise, while theoretically true, is not practicable in reality. To compute the realistic trajectory (more correctly: to estimate the probability of all the transitions between all the states to build up the PDF) would require multiple universe-lifetimes to calculate with enough accuracy to replace what an xFEL can tell you.

It's not just about building bigger computers. If it were, we would make that investment. If you look at the QED and QC calculations done on the world's largest supercomputers, they're not that inspiring.