Why would you make Aegis more capable of tanking physical things? That's not it's purpose. It's already a very good physical shield compared to the non-Relic/Empyrean and it fills a perfect role as a magic damage mitigation shield.
You're asking for a relic to be adjusted because it doesn't fulfill a role as well as an empyrean does, when by comparison, the Empyrean doesn't fulfill a certain role (magic damage mitigation) as well when compared to Aegis.
Should we give Ochain the -MDT so we make it a good shield capable against magic casting mobs?
No.