I understand your point, and in simple terms, you're correct...an easy to understand analogy. Though, in pro ball, AA is the true developmental league. It's loaded with the young, rising stars and where they see the most growth and challenge among similar-aged peers. AAA is a bit more like a warming bench for MLB-ready players, but many are fringe big leaguers. Your young prospects will step from AA to AAA, but the ones on the fast track will oftentimes spend more time in AA and less in AAA as long as everything correlates once they hit AAA.
Like others have said, the ECHL is pretty substantial step down. They're great players, but the AHL is the true development league for the NHL for those that need actually need development outside of the parent organization and aren't NHL ready right away.