>The aerostat will rise. It will float higher in the atmosphere, with decreased pressure around it. It will expand. It will then rise still further.
Now explain weather balloons. Why don't they rise to infinite altitude?
Like I said, the numbers are all wonky, but the principals are the same.
If there is too little mass for the amount of bouncy just compress your gas and hold some reserve buoyancy/balloons to inflate if you expect to be able to deal with rapidly increasing mass.
Weather balloons reach a stable equilibrium altitude because 1) they're designed to expand as they rise (at quite an impressive ratio) and 2) they're not suddenly gaining or losing 100s of tonnes of mass.
At least one if not both those prereqs is missing from the observed case. Though discussing the matter further has lost virtually all appeal.