Theymos has said he'll be monitoring it closely and adding more merit sources as he sees fit, hopefully that will be the case and we won't end up in a scenario where ranks are much much harder to come by. I don't think anyone will have a major issue with a slower rate of progression, even if it was halved or so, but if it's 50 times or more then it's a bit ridiculous.
Going by your numbers:
Another viewpoint of math is that this is essentially asking the current 50 merit sources to classify 451,274 posts per month or 15024 (451274 / 30)
posts per day per merit source. This gives each merit source about 5.75 seconds per post in a 24 hour work period to read / judge / and assign a merit score.
On top of this the merit score doesn't really discourage the old timers from posting shitposts either for campaign purposes. The lower ranks still have to read those..