who/what "ate up" (subsidized) the cost before if 15 SP could grant that much user activity? currently 15 SP isn't enough to grow an account.
Oh, that I can tell you without hesitation – the original 15 SP came out of the vast, vast, vast (did I mention vast?) @steem SP wallet. They tapped 15 SP to be delegated to every new account creation and of the daily creation costs barely moved the needle on how much SP remained in the pool. Eventually, for established accounts they stopped delegating that initial 15 SP and it returned to their wallet.
Now, anyone can create accounts – but the SP also has to come out of their pocket. Which is fine and even reasonable, but along with this requirement has come the fact that the minimum viable SP for basic engagement on the platform has gone up so every digital application that wants to on board users needs to have at least as much SP investment as they would like to bring users on at a reasonable pace times 20 or 30 – and I hope that you didn't want to actually use any of that investment money for paying yourselves, paying for services, or in fact anything else, because you won't be able to. You'll need it as the cushion to onboard new users.
And unlike Steem Inc., you won't have a vast reservoir of "ninja mined" (a term which I think is unsuitable but which others seem to like quite a lot) SP to bring users on with. You are going to have to acquire that SP, probably by bringing in real fiat currency in exchange.
And that is going to suck if you were thinking of developing a digital application on the steem blockchain.
This is a bigger problem than it looks like, even if you assume that the minimum viable SP for an account is 5 to 10 rather than the more reasonable 20 to 30 it looks like right now. I can't think of many projects with enough SP and enough SP coming in per month per new user to be able to afford to deploy for any users who don't already have accounts on the steem blockchain.
Imagine you're in that position. What would you decide?