As a Platinum Medallion on Delta for the second straight year (270 paid Delta flights in two years), I would like to remind you that all Medallion level upgrades are a PERK (see also BONUS, FREEBEE, GIFT) and not a privilege... Why should an airline reward low-paying frequent travelers before those of us who are higher-paying?
L-class fares require one to buy a ticket quite early... This also would allow a Platinum Medallion to upgrade quite early, taking seats that are currently given to those of us who pay more. Who would you rather have a bit tweaked at you? One who usually pays the lowest fare, or one who pays more?
It's simple economics... Delta gets more money from non-L fare passengers and, if forced to make a choice, would rather keep them happy. If you've been paying attention, you've noted that First is FULL on 98+% of its flights.
Maybe with an L-fare, they could let you upgrade at the gate if a seat would otherwise go unsold...
And I also look out for the company's $, flying on an L-fare when it is available, so please don't even go there.