> The point is that there aren't many complex tasks were fable delivers a significant value increase over cheaper models.

Strong disagree on this. Any decently complicated task like a refactor is going to be more likely to be solved by Fable than by Gemma 3B or whatever.

I have personally tried to use Sonnet over Opus for tasks and Sonnet gets things right sometimes and at other times I wish I had just paid higher.

This is the standard pattern I keep seeing and I can have a bet with you that it would stay like this.