I set my mother up with a set of Panasonic DECT6 phones about 7 years ago, and most of them died about 3 years ago. I say died, but really it was slower than that, more like the battery life got progressively shorter over the last few months or so, and the volume started to go. I felt they lasted pretty long considering how cheaply they're made

I say just go with a Panasonic set and replace it when it dies. 3-4 years is pretty long for this kind of device these days. Heck, you can get a set of five phones for $120, and if they last four years that's $2.50 per month.