Not sure what you mean by "objectively", but I have a few simple criteria that I turn to before I decide to replace a device:

1. Can I easily upgrade it (new hard drive, more memory, change the battery, install another operating system)?

2. Is the cost of repairing/upgrading the device commensurate with the amount of use I'm going to get out of it post repair/upgrade?

3. Do I really need to keep it going or buy a replacement?