If you have a gymnast who thinks she will eventually be trying to compete all the way up through level 9/10 and you want to pick one gym for the whole journey, you want to know a) the gym has the capacity to support high-level gymnasts AND b) the gym has the capacity to get many lower-level gymnasts to a high level. From my experience coaching, I've found there are coaches who are great with high-level skills but not necessarily good at teaching the intermediate gymnasts around levels 3-5. There are also coaches who are awesome with strong basics and conditioning at levels 3-5 but don't have a lot of technical knowledge about higher-level skills. Some programs are also more focused on developing one or the other - and many programs split up the coaching staff and practice times so the compulsories/optionals divide can be pretty palpable.
If you see a program where there are a lot of successful optionals who have been there for a long time, that suggests that the gym is working out well for both lower and higher levels - actually, what it really suggests is that the lower-level program was working out well 5-10 years ago, since that's when the current high-level gymnasts would have been in the compulsory program. And not every level 3 gymnast is going to have the skill, the resources, and the desire to make it up to level 9 or 10 - and attracting higher-level gymnasts from other gyms shouldn't reflect poorly. So looking at the number of "home-grown" optionals doesn't tell you exactly what you might want to know, but it's something to look at.