I agree. While building the initial product takes time in and of itself, adding features to any project takes an even greater effort IMO.
You have to think about things such as adding a new feature which works well with the rest of the site, add the new code without making a mess, change the existing implementation of a feature 'cause you realize your first implementation was not the best way to do it, or that it does not work so well with the new feature etc.
All this time and effort should be counted in - putting out an initial release is good, but you still have a lot of work left to do.
You have to think about things such as adding a new feature which works well with the rest of the site, add the new code without making a mess, change the existing implementation of a feature 'cause you realize your first implementation was not the best way to do it, or that it does not work so well with the new feature etc.
All this time and effort should be counted in - putting out an initial release is good, but you still have a lot of work left to do.