While the specific number is still debatable (250 MV)
Agreed. It is just a proposal, and a simple number chosen for conversation sake. Details like this can (and probably would) be tweaked if the proposal were to actually be implemented.
The original idea that @snowflake proposed actually had the amount being set in dollars (rather than vests) so that if 5 years from now STEEM is worth $100 - we don't have the same problem all over again. (It would be a good problem to have, but might as well solve it once and be done with it.)
In addition to this, I would like to suggest applying n*log(n), which is less exponential than n^2 to the reward system. 250 MV vs. 1 MV is a large gap if we keep n^2.
I haven't given this a lot of thought yet, but I have been thinking about it since you brought it up last night. My initial reaction is that I like it!