The question doesn't start at why do we need to reduce posting rewards. The question should be, why do we need to pay people to like us - because they only like us when we pay them, and then don't, when we stop paying.

Because increasing the debt burden through raising the interest on HBD, means that we have to take it from somewhere. If the interest on the HBD was at 10% for instance, then perhaps there would be no need to take from the reward pool.

also, I do not see how people who buy HBD (with their BTC->Hive->HBD?) beat on those who back HBD value by working?
those buying HBD also back HBD value by buying -> directly putting in value

Suppose we have two investors, one buys HBD and one buys HP. Both buy the same USD value of each.

They are putting in value for the HBD that they bought. Not the 20% extra that they get on the interest.

People who buy Hive, must (in addition to putting in the same money) work 24/7 to curate effectively to get at best 12% return, put up with 13 week power down duration risk, as well as market price volatility.

Both at point of purchase, are putting in value, but one maintains or attempts to grow it's value through (hopefully) good resource allocation / decision making on governance, and the other collects selfishly for no further work, and with much lower downside risk compared to the other.

The comparison is not even close.