Obviously, the value of STEEM is determined by one thing: the price people pay for it in exchanges.
Recently, I came to this realization that the more SP an account has, the bigger part of its vested STEEM its owner can redeem by powering it down, but it can never get to the entire stake, since powering down to 0, or even to some positive values (above 0) will render the account useless until enough power is delegated to it as a form of a restart.
And even that SP has to come from somewhere that paid for it (not delegated) because delegated STEEM can not be delegated by the receiving end of the delegation.
This means, that some STEEM has to be paid for, and the more accounts the bigger the paid for amount will have to be in the future.
More accounts means a lower average amount of STEEM per account, and a bigger average portion of unredeemable STEEM in each account, which translates to less liquid STEEM, which translates to both a lower supply of STEEM and a lower velocity of circulating STEEM, all other factors being equal (remaining unchanged) obviously.
So in a way, I might have partially answered to my own question from the past, why do some whales rent their Steem by delegation instead of using it to upvote themselves.
By doing the much less profitable in the short term, which is renting, they give potential new users hope in joining, and current users hope in staying.
This way, while decreasing their own future income in either STEEM or SBD terms, they increase the values of both STEEM and SBDs, since they increase demand for them.
Another source of the value of STEEM is in its ability to control its inflationary flows, both author (and the much lesser curation) rewards for active members, and producer rewards for witnesses.
Ironically, STEEM's inflation causes an arms-race which causes some increase in its value in contradiction to the obvious decrease in its value due to inflation.
Downvotes/flags are another contribution to the value of STEEM, due to their decrease of amount of future liquid STEEM.