There's a concept in engineering called "Reißlänge" (≈ breaking length), which is how long a cable of a certain material can be before it would rip due to its own weight if suspended by one end [0].
The values for some materials are actually much higher than I remembered, with Kevlar reaching ~250 km. From a purely hold-its-own-weight perspective it seems like it might actually be possible!
Obviously there's a ton of other problems that wouldn't make it feasible, the first I can think of is the weight of a 50 km tether wouldn't allow the balloon to even take off. I'm sure you can think of another million reasons it wouldn't work.