By successful, I mean the Mormon church succeeds at convincing people of its truthfullness. It succeeds at gaining converts, retaining them through out their lives, and convincing them to teach their families to accept the doctrines as true.
Your argument about after-life conditions is an important one. Someone suggested to me that disgruntled former theists should sue their churches in court to be refunded the donations they gave throughout their time in the church based on the failure of the church to demonstrate fulfillment of their committment to improve after-life conditions. I wonder how that will go in court. I would like to see the atheist/agnostic judge rule that the donations were voluntary and so not subject to refund, but, in exchange for that positive verdict for the churches, would order the churches to cease claiming that they could improve one's afterlife judgement. That would be quite a thing, I think.
I would like to see tests done whether on earth blessings are improved by making donations to a church.
In the future I would like to hear: "Bishop, why should I obey the commandments?" "Because God will bless you in the afterlife, oops I can't legally say that. Because God wants you to." "Will I obtain the celestial kingdom if I obey?" "I legally can't confirm that." "Then why should I pay tithing?" "Because God wants you to, but I can't say because you'll be blessed."
If religious claims were forced to comply with the same rigorous requirements as scientific claims are you'd see organized religion practiced very differently (if at all).
My mother taught me that the LDS church makes bad men good and good men better. In many cases I think that's true. The church emphasizes family, health, and such things a little bit more than society does. In most cases, as with all religions, Mormonism teaches people to depend too much on authority and resist becoming free-thinkers.
There is a bad consequence that often occurs because the Mormons teach their men that to be obedient to the doctrines is more important than prospering materialistically. Magnifying their church calling is more important than advancing at work. Because of this many LDS men accept mediocrity in their professional lives. Women are taught to pick a "spiritual" man (meaning one who obeys the doctrines) over one who is prosperous. When these women struggle with unnecessarily low incomes (partly due to 10%+ in church donations), their church chastizes them for complaining about things that don't really matter.
An atheist has argued that the Roman leaders adopted Christianity because they could use it to convince the citizens to suffer with poor earthly conditions in exchange for a happy after-life. Unfortunately, no one was able to show that Christians aren't any better off in the afterlife.