The end game will be to much debt, debt limit reached and hbd worth less, meanining it cant last long. Also not sure will any fee save us from this scenario

It will if it deters it from happening in the first place. Too much debt and HBD being worth less isn't a desirable equilibrium.

It will be needed some serious capital to do this manipulation

With small enough or no fee you don't really need to move it a lot. Any imbalanced price fluctuation turns into profit. It may be small but the time period is also small so the rate of return may be significant. With fee the possibility of this decreases. In part for similar reasons I would like to see some of the fee moved to the HBD->HIVE conversion.