Hasn't "too much capacity" existed since the moon missions? It was too expensive , but most other missions did not need those huge rockets. It wasn't utilized either. Remains to be seen if spaceX's capacity will be utilized.
It was huge capacity, but at a higher cost, so you try to get your payload to fit on the cheapest rocket you can.
Starship turns everything upside down and, suddenly, makes the cheapest rocket a Saturn-V class heavy booster. With it, it's cheaper to add a huge kick stage to your Neptune probe to make it get there faster, put more solar panels so you don't need to deal with compliance around an RTG, or just use steel for structural elements (because why not?), and so on.
The question is price. If capacity is cheap, then you don't need to fill it with a billion dollar sat to justify the launch. We can start sending cheaper things to space, such as product/medical design labs to fill the cargo bay.
