I think that we didn't have conversions in BOTH ways while we were on STEEM, probably on purpose... to protect the BASE token (STEEM), and not the stable token... Atm, we are doing it another way around...

We added the conversion from HIVE to HBD to put a ceiling on the price of HBD. You mean not having the conversion would stop printing more HBD and have less debt? I assume this is what you mean.
We saw what happened when SBD went up in price to like $7 and that created way more inflation than what we have here.
Because the chain assumes HBD is $1, it rewards the authors based on that. So if HBD was $10 in real life, that would mean the debt is 10 times higher than what blockchain assumes it to be and it keeps printing more HBD and the debt goes way above the haircut rule and when the haircut rule finally gets activated or the price of HBD drops to $1, the chain has to pay 10 times more debt than it would have originally paid.

So in a nutshell the added conversion is a protection against this which we already saw happen years ago on steem and sbd.