Refrigerator Lifetime?

So my Maytag top freezer frig is 15+ years old, still keeping everything cold, but the compressor is sounding a little less than happy. It's just not as quiet as it used to be. Very subjective. You helped me replace the Cold Control Thermostat two years ago: http://www.applianceblog.com/mainfor...-Run-Now-Works

My questions:

1. How long should I expect it to last?
2. How/what fails on a refrigerator?
3. Should I replace it before it fails?
4. Thinking of getting a Kenmore Side-by-side: http://www.sears.com/shc/s/p_10153_1...liances-active Bit more efficient than my current Maytag, huge freezer, no ghost loads (mechanical thermostat). Thoughts?

I'm solar electric, so ghost loads are very important (I have an inline switch on every appliance with a ghost load), as well as efficiency.

Thanks for your time. :)