> "and after six months of getting him up to speed on a fairly complex fintech application he resigned and went to a a random web development job that paid more."
The immediate payjump from hopping from bottom-of-the-rung where you're at to some random CRUD web dev job, I'd wager is far outweighed by the potential returns for senior experience in Fintech, specifically.
That's a much harder niche to get into than building CRUD apps from what I hear.
Not six months, but about a year for each.