I was wondering why they put Miss Croner in when they could have used Miss Miller, who seems relegated to a brief cameo character nowadays, although Miss Croner is a lot more wikid and sinister than Miss Miller (like tricking the kids into searching for a cat) but they could still do the funny "crazy old person" comedy. Also, they could then have the Chipettes move in, giving them their own "Dave" counterpart/mother figure (I like Dave). Seeing as they would be neighbors (Chippettes still in close proximity) although Miss Croner lives in the opposite direction of where Miss Miller lived in the 80's series. Speaking of which who lives next door to the Chipmunks on the opposite direction from Miss Croner?
Perhaps they wanted a bigger distinction between the 80s series and ALVINNN!!! so they created a new crazy old character while giving Miss Miller more for the "feels." Though, Miss Croner doesn't really replace Miss Miller as Miss Miller is still in the babysitter role. Looking back, Miss Miller appeared as a dominate role later in the series so it's possible (not sure how likely) the girls could have a similar experience and move from their treehouse. How interesting would it be if The Chipettes moved in with crabby Miss Croner?
I don't think Ms. Croner's demeanor would be condusive to caring for children, but you could say the same thing about Ms. Miller.
It would legally be difficult to do because Dave likely legally adopted the girls (otherwise they wouldn't have been able to stay at his house before they had the treehouse). Having them move to someone else's home without him would result in action from child protective services, who are presumably more competent than Officer Dangus.
In the 80s series, the girls weren't likely even legally registered before they went to live with Ms. Miller (they were babies when they came to America, and likely didn't know how to fill out the paperwork), lived in a forest adjacent to the town (which is in a sparsely inhabited suburbs), and it was the 1980s, therefore, they could get away with it. I'm not sure how they got enrolled in school or got smuggled abroad for tour dates.
In the current series, the area they live in is heavily populated, and they live in their adoptive father's backyard. It's also the modern era, with more parental suprivision, and checking for legal paperwork more often.
The tree the treehouse is in straddles between Miss Miller and Dave's properties, although the trunk is on Miss Miller's side. If the girls were adopted by Miss Miller, then why would they have lived in Dave's house before the treehouse got built, which was before Miss Miller even moved to that house?
I don't think Miss Croner is, in any way, an attempt to replace Miss Miller.
The current series is far more fleshed out than the '80s-'90s show both in terms of stories and characters.
The likelihood is just that more regular characters, in general, were required than before.
Also, Miss Croner's feisty character, fierce indepenence and use of slang is a good contrast to the dotty, sweet-ol'-lady character of Miss Miller. A way of showing that not all elderly people are the same and that some are, indeed, fiercely independent and young at heart.