That doesn't explain software house behavior at all. Development organizations take shortcuts and intentionally incur technical debt in order to hit the market window. If you don't have short-term sales success then there's no revenue to fund long-term improvements.
And how can you design "decent systems" that are adaptable to change? It's impossible to reliably predict what changes will be needed. Most software products evolve in entirely unpredictable ways. There are some general software engineering best practices for modularity and layering but beyond that you're getting into YAGNI territory.
I think that a mature organization has enough of a reputation to not need to do that. This approach to running an agency is necessary for the bootstrapping phase but beyond that it will become a burden very quickly.
Best practices are pretty much what I'm talking about. It may be impossible to predict what changes are needed but it's easy to predict that there will be changes.
And how can you design "decent systems" that are adaptable to change? It's impossible to reliably predict what changes will be needed. Most software products evolve in entirely unpredictable ways. There are some general software engineering best practices for modularity and layering but beyond that you're getting into YAGNI territory.