There are issues with replacing phones, versus replacing parts like batteries.

Having to learn a new phone UI isn't ideal - but worth it if you're moving to something easier/better.

Having to replace all the phones even if only one or two have died - if the new ones aren't compatible with the base you're currently using. "DECT" is of course supposed to solve this.

Re-entering all your contact information into the new phone(s). Surprisingly, there are few products on the market that make this even remotely easy/simple. With my Siemens it has BlueTooth (which is buggy/unusable) or USB from my Mac's address book, using of course their own proprietary software. I just need to do a single phone and then I can replicate that, without duplicates to the other phones.

The state of home phones is pretty stagnant when you compare to mobiles. Of course if you want to overcomplicate a home phone and run Android on it, Archos has you covered:

http://www.archos.com/products/home/archos_35_shp/index.html?country=gb&lang=en

It does have an excellent feature which I also have with Siemens. The base doesn't have a phone cradle. Which means even with a one phone installation, you can put the phone and base in two completely different locations.