Airmann, what would your colleague feel like if you did managed DES, and assuming you were below the VNAV profile, the FMGS/FD did -500 fpm? That's how the VNAV is re-designed on FMS2 that has been the manufactured standard for more than 10 years already
I share the sentiment of posters above. UK, where mandated, stipulates CDA below TL as one of the reasons to use less than 500 fpm, which is the de-facto minimum required rate in their airspace.