7 years and 4 different companies is why your perspective is skewed. You’ve never seen a system mature at that job change rate. Therefore you have never really had to deal with decisions that impact long term which is where architecture matters.
Secondly, you can't know he hasn't worked on mature systems (he didn't say). Most people learn by working with a variety of systems that are at different stages (greenfield to twilight), and more importantly with different quality (poor versus good architecture/systems/teams).
A long period working on one good system may help a few people learn, but from what I have seen, it often doesn't (I have worked with crap programmers on mature systems).
Hopefully GP answers for themselves.