It all depends on how hands on you are willing to be.

If you are using the model to write to code faster with extensive human oversight you can develop a lot faster using the non-frontier models. I was doing that extensively last summer.

But now my thought process is I want to focus on architecture and product direction. I have not seen Sonnet level models be capable of performing autonomously enough to take a feature end to end reliably enough to be completely hands off. In fact there a many cases where Opus will fail as well where Fable will succeed.

Of course that is not to say that Fable will always do things correctly. It will happily take an under-specified problem statement and happily use up all of your usage to build the wrong thing, while Opus at least recently stops constantly to check in.