Tithing may be defined as “voluntary” within the church circles, but the temple recommend negates that through tithing. How? Tithing is a mandatory “condition” placed upon members in order for them to secure a temple recommend and thus take out their endowments, which then is supposed to help ensure their celestial glory.
It’s “extortion” in this sense because mormons are taught that of the 3 degrees of glory, only the celestial kingdom is reserved for those who have been “obedient” to the covenants and oaths taken in the temple.
So, if one wants to be able to dwell in the presence of the mormon god, one needs a temple recommend, if one wants a temple recommend, one needs to be a full tithe payer.
How is that not “extortion”?
As to “obligation”, open books lends credibility to an organization. It demonstrates to the flock that their monies are being used for the purposes intended.
“Prospering” in this sense is the antithesis of what should be motivating a “true” church.
Saving souls, it’s what Jesus does, not a “prosperous church”.
Yet nobody is kicked out of the church for not having a temple recommend, that is voluntary too, and once a person gets their endowments they are not excommunicated or have their temple blessings revoked for just stopping paying tithing. Likewise, there are members who are currently unable to get a recommend due to an addiction or some sins that have to dealt with who still pay tithing, recognizing that it is a commandment of God.
BTW: a prosperous church is in a way better position to save more souls than a broke one.