I'm PM and get upgraded most of the time, so I find it hard to believe that boarding DM with PREM will thin the SKY boarding zone herd very much. It seems more like a DM consolation prize for missed upgrades when they occur.
I've heard that AS is doing something similar in regards to rewarding non-elite customers with priority boarding in return for giving up their (already huge) carry on bags. This will ease the pain for those boarding in zone 2 and higher, but will likely just further frustrate FO and DL Amex customers looking for an elite boarding experience in zone 1.
Both changes appear to be putting the same coins in new piles. Just my two cents.