Yeah I agree the stablecoin isn't required to work, but it would be a huge boost, and I also agree that the overall platform needs a lot of chicken-and-egg work.
You're also right that creators would probably sell the SBD for fiat, but still not necessarily right away, especially if SBD were paying some competitive level of interest (even 1/2-1% is probably attractive). People looking to earn rather than speculate are much more likely to hold SBD than STEEM if they don't necessarily need the funds right away, and many don't.
Finally, creators selling SBD could be overall offset by subscribers buying SBD. The key is to have willingness to hold for more than a millisecond. That confidence translate to value entrusted to the system.
But yeah, it all depends on building confidence in the platform as a whole.
What do you think about implementing the possibility to power up SBD as well as Steem ?
Meaning that the 'powered up' SBD should not be taken into account for the haircut calculating.
Instead it should be taken into account for the calculating of vesting_shares.
Interesting idea, but I fear it would not be a high priority for blockchain development for quite some time. We can keep it in mind though, and revisit after more pressing issues are addressed.