OTWO, you have it right. It's all about the money.
Like any business they need to balance how much or how little they can require from the R&F members AND still make the most profit.
They seem to be tinkering with their formula in order to increase membership and maximize profit. Or they may wish to maintain only a hardcore following of dubs and see how profitable that can be.
If they can get the same cashflow from a 2 day convention and 2 1-day assemblies they'll increase their profits quite a bit. (keeping in mind that they must maintain mental control over their members so Sunday only worship probably wouldn't work).
I wouldn't be surprised if at some point they start pushing how good and helpful the tithing system in Israel was. I'd say it would never be mandatory but I can see it being recommended. It could turn into a prestige thing. "I give 8%" or "I give 10%, how about you?" - something like that.