The reason that I mentioned that is that in a retcon that was published a few months later, Deanna had a similar reaction to Mira and Wilf's super-scary conversation about how gets what and where they end up being laid to rest after they pass away; what really made Deanna's whining about being scared is not just because she's thirty but that she has power of attorney over her parents should things go wrong. This tells me that after watching their youngest daughter act like a small child confronting the boogey-man when they tried to get her to face up to a sad reality, either Wilf or Mira decided that their new retirement plan should be summarized by the phrase "Get the immature idiot Deanna to give the big, scary power of attorney to Andrea because she acts like an adult."
There are, of course, other reasons that aren't related to not letting an immature fool screw up their future because her reaction to the future is to stop up her ears and pretend to not listen to do this. The first reason would be to avoid watching their legacy get frittered away stupidly so that John and Elly might live comfortably at their expense. The second is to avoid becoming the same sort of jerks the Pattersons are. Mira might have a lot of bad qualities but one of them isn't any sort of desire to become known as one of those old fossils who made their kids their servants. It would seem to her to be a nasty trick to play on Deanna to raise her to become independent only to become her unwilling servant.