Emirates use DEC probably because it is cheaper than upgrade an FO. It is faster also. If you upgrade an FO you need to form an other FO. 2 courses. Probably some FO are suitable for upgrade and some are not. It is normal in any company that some FO even that they qualify they fail upgrade courses.
The real thing is that emirates can hire DEC legally if they want. And you know it. The problem is not a narrow body Capt DEC. They are qualified. It is a more relaxed job than 20 days on the field. If you attack them is pilot against pilot. Poor against poor. The problem is management.