Agreed, but how do we acheive it?

I looked long and hard at this issue when I developed my course on planetary science. There seem to be a few "rules" in place in our Solar System (though the evidence from other systems suggests that we are a mere subset of all possible parameter space).

1) Bigger gas giants have more moons
2) Bigger gas giants have bigger Moons
3) They are still very small

Nothing in our Solar system is a moon AND is Mars or Mercury sized. Even those are too damn small to get interesting for life. (Even IF they were in the Habitable Zone*)

Furthermore, the hints as to scaling laws for size of Moon vs size of gas giant strongly suggest that you don't normally get Earth-sized moons, unless the gas giant is already well into brown dwarf or low-mass star status.

I admit that we know almost nothing in this field. I agree that it is likely there will be exceptions. But I suggest strongly that they will be very rare indeed (no more than 10^9 in our entire Universe).

* I here declare that whenever I talk about an HZ, I mean "A zone where complex surface life could exist and develop to a point where a technological civilisation is not just possible, but evident through deliberate or accidental electromagnetic radiation patterns. To do this, a wide range of basic parameters have to be so adjusted that the world in question would be judged as totally Alien, yet also strangely familiar."

No point being habitable if you don't inhabit...