Is it better to drive off the small accounts with a tax?

I'm not following. N^2 had a massively bigger 'tax' on small accounts than the current curve, which is rather subtle. In the case of n^2 it was often 10000x or more. Even with the cap, it was still quite significant (when linear came into play the voting power of the small accounts dramatically increased even relative to the 'whale experiment' conditions).

The ninjamine can only be sold once.
When does that run out?

Steemit still has at least 30 million SP, though probably more as they moved some to 'hidden' accounts and may or may not have sold it. They have paused selling but when they were selling it was 800k/month, so that is at least a few more years of selling at that rate, should they resume.

All other early miners (who were all minuscule by comparison, even taken as a combined total) have either long since sold or apparently have little intention to sell. No one else besides Steemit started out with so much as to make it impractical, or at least economically questionable, to sell it all within 3-4 years. Arguably Steemit has sold a bit too fast, if one views the price action as partially a result of all the selling (which I do).