Why are so many people convinced there'll be a cash-in that night? Here's why there definitely won't be:
1. Becky is defending both titles AND someone will win the briefcase, so WWE exits the night with at least two women's-related headlines. Have someone cash-in and it's just overkill.
2. The female winner cashed in immediately last year.
3. WWE is desperate for storylines and interest AND they just introduced the stupid wild card business, which allows both MITB winners to float between shows. The briefcase is such an easy storytelling tool that there's no way, in their current desperate state, that they'll squander one on the same night it's introduced.
Much of what you say has merit but there are a couple of issues to consider.
1. Becky having both titles moving forward undermines WWE's ability to create multiple storylines for the women. You'd have Becky holding both belts feuding with someone from each brand while also dealing with the potential cash in. Makes sense to have her drop one of the titles in order to expand things a bit.
2. That said, WWE has proven incapable of doing multiple storylines with the women (hell look how they handled the SD title by taking it from Asuka leading to Mania in order to focus on a single storyline). So it may not be clear that they are desperate for storylines, at least as far as they may see it.
3. WWE just loves the heat and shock factor. We know they rather create moments as opposed to long term out and well planned stories. So them deciding to go ahead with the cash in would be something they do.
We'll see of course, I think the key, at least in my view, is Becky dropping one of the titles. After that they could hold off the cash in for later.