Yes, you're right, that was an over-simplification on my part. I think it's a combination of time plus external factors, usually lots of encounters with other languages. English is incredibly pared down because it's encountered so many other languages/ cultures which have forced it to simplify (I think for e.g. some of the simplifying influences of the Viking invasions are relatively easy to trace).
I've also heard Spanish referred to as "soldier's Latin" - certainly it has a reputation of being grammatically more straightforward than the other Romance languages. (I find French relatively easy grammatically - Italian is a lot harder for me.)