I don't see that paying a fee is making it a closed system. If someone really wants their game to be greenlighted then they will find a way to get the money for the fee (either from themselves or asking friends/family/people interested in the game) but once they've paid it - it's open.
People who want to spam the greenlight are much less likely to do so when there's a fee there (and the fee is about right to stop that - any lower and people would just do it jokingly).
The other option to stop the spam is to have moderators (and or the public) vet/approve the submissions - which then *does* make it a closed system as you're essentially letting a person (or people) decide whether it should be on Steam. If someone posted HL3 to greenlight as a joke then of course it would get a shitton of votes but it still wouldn't be an authentic submission.
If you don't like the fee as a developer, advertise/sell your game elsewhere until you get the money.
As for:
the barrier to entry should be making a product and selling it. you know capitalism. the fee is just a dumb extra requirement. if someone goes to the trouble of making something dumb, and other people buy it who are you to judge?
Because I want to see games in Greenlight that I genuinely might be interested in playing. So I expect to see a certain level of quality in what's there. I don't want greenlight to be flooded with fictional and/or low-quality submissions.