Just read through the thread again, and I think I understand EK's point.

1. You claim a fare difference of 1500AED, This is based on the price you were shown online at the time of booking? This might be the difference between J Saver and F Flex (you mention being F Flex on the way there and J Saver return however EK mail confirms you are paid F so I assume you are F Flex return?).

2. At the time of the downgrade you were possibly (you don't mention explcitily as far as I can tell) rebooked to J Flex or Flex Plus

3. My assumption would be that EK rebooks you on the same ticket terms (i.e. change fee, cancellation fee, etc.) when the aircaft configuration changes, so if you're on F Flex your ticket is changed to J Flex.

4. Thus your refund is the difference between a F Flex that you paid in August and a J Flex at the time of the downgrade (not optimal, obviously, as the price would've potentially changed by the time of the downgrade), but the refund is not the difference between F Flex and the lowest J fare from the day you made the booking.

5. By that logic, EK is probably correct in their calculation, though that doesn't mean it's "right"

EDIT: And 50,000 Skywards miles at an approximate value of 1.5cpm gives you around 2,800 AED in value, so all in all that's 3,100 AED, but of course the miles can't be used as cash though I'd argue the significant amount thereof over the cash refund might make up for that. Even though your utility for the miles may be marginal as you state you don't need them.