Back to the point, the only difference to hive is, that the underlying asset becomes not inflated that much, and the stable coin becomes worthless ( not stable).

That's a huge difference.

And in the normal scheme of things, the description of HBD value during a haircut scenario is "worth less", not worthless.

For the most part, HBD holders can mitigate a lot of potential risk of a loss due to the haircut rule by monitoring the current haircut ratio percentage and exiting their HBD position if the percentage goes too high for their comfort zone (gets too close to the haircut ratio). I've used this strategy a few times myself very successfully.

so if 30% of the 30% would be converted and again we end up with 50% if price would be the same.

I don't view this scenario the way you do. If all the HBD got converted to Hive, expanding the supply by 30%, and the price stayed stable for long enough for a lot more HBD to be created to potentially repeat the cycle, then I think Hive successfully weathered the increase in supply.