I can understand the oven problem. It may not be that it's "embarrassingly dirty." Embarrassingly dirty could be code for she can't use it without getting smoke, and can't wait most of a year to fix it. It may be, like mine, that it's gotten to the point where it's smoking. We've switched off the heat for the summer, but it was cold this morning and my husband turned on the oven to warm up the kitchen. That eventually set off the smoke alarm - which woke me up. I'll be cleaning the oven today. :)
But at age 70, there could well be physical limitations that make getting down on hands and knees and scrubbing it impractical. My mom, in her 70's, has had knee operations and isn't supposed to be on her knees at all. I can understand how, to a 70 year old, cleaning an oven might be the same as running a half marathon would be to me at the moment. Doable maybe? but not something I'd really consider because I'd be in severe pain for days afterwards.
Your giving her the benefit of the doubt speaks well of you. :)
In this case, though, there's no real problem with the oven--she barely uses it, and when we were visiting late last year I didn't see any real issues with it; it was mildly in need of a clean, but nothing horrible or dangerous. I think it's may just be a bit of germ-o-phobia; she once threw out most of an entire package of perfectly good disposable cups that she brought with her to a stay in a vacation property because she was concerned about taking them back home with her because they had been "exposed" (i.e. the package had been sitting open in the kitchen of the rental for a few days).
Mobility issues are a distant "maybe"; she's fairly active and gets up and down stairs well in her condo, and sits on the floor to play with her grandchildren, but I can see that bending over for a long period of time might be a challenge.
Even if these things were issues, though, I'm just mystified at the whole idea of replacing the oven because it's dirty--she could pay a maid service $50 or $100 or whatever a single visit would be and get the oven cleaned, instead of spending many times that on a new oven. There's no hoarding issue where she would be embarrassed about someone coming to the condo; it's immaculate and well-cared-for. So I'm still in "mind boggled" mode over it. ;-)
Edited to add: maybe she just wants a new oven because she just wants a new oven. :)