You don’t get to actually save on the expensive bits of rockets.
Within a small rounding error you need just as many and just as large rocket engines. Sure, you save a little fuel, but that’s the cheap bit. Structurally you need to handle dangling from the balloon, which means your cargo capacity is about the same size and possibly worse.
On top of that you need to both pay for the balloon and extra R&D for a complex system. If everything worked perfectly you might see a small savings per launch except the market is tiny so just breaking even on R&D is difficult. On top of that the more complex launches mean more can go wrong, meaning higher insurance costs.
Finally this is not the 80’s. Compare it to SpaceX’s reusable first stage that gives not just altitude but also velocity. Something like that is a huge advantage reducing the number of expensive bits needed to manufacture.
PS: As to dangling. When you fuel a rocket on the launch pad all the stages sit on top of each other. This means you can build a rocket that would break if sitting on it’s side, because all the forces are down before and after the engine fires.
