Also, I've heard estimates from as little as 25% to much more than that as the percentage of members for whom the Church has completely lost contact. That is, there is some significant number of members whose membership is so tenuous that the Church has nothing but a record. There is no address, no way to contact them, and no information about whether they are living or deceased. These people continue to be counted in the total membership number despite their complete absence of contact with or from the Church.
With the significantly lower fertility rate, and an aging population, the Church's future will not be anything like the projections of Professor Stark. That is, unless something changes.
I agree that there may be many reasons for the decline. However, the most prominent of reasons in my view is the de-emphasis on doctrine. As a convert to the Church I know what attracted me to become a Mormon. It had nothing to do with the formulaic discussions of the missionaries, slick marketing or good arguments. It had to do with doctrine. I DIDN'T WANT to be a Mormon. Quite the contrary. But I knew I should become a Mormon because their doctrines came from God and answered questions other faiths could not begin to answer.
I'd like to see the trend return to a dramatic increase of numbers. In fact, I think there are many millions in the United States alone who are only kept from the truth because they do not know where to find it. (D&C 123: 12.) We won't attract them to the Church until we begin again to emphasize doctrine.