I think a dive computer should function without problems for 20 years. I remember first multi gas dc coming out to the market 20 years ago, since than, there are not much of technological advances except displays and some "nice to have" but not mandatory features. It is very difficult to meet 20 years requirement with a rechargeable and not user replaceable lithium battery. If you feel 10 years usage is adequate, a rechargeable should be fine.
You think it should go 20 years without ever being serviced in any way?