The value of your service to a user isn't going to increase simply due to the passage of time or growth of your site. If they wouldn't pay for it on day 1, the odds are they won't pay for it after 2 years of using the service. If, in the process of trying to monetize your service, you "hold hostage" a portion of the value that the user has contributed to your site, they are going to be pissed.
This has all happened many times now. I think companies trying this route in the first place will be easier to change than peoples' reaction to it.