It's called business!
The DD situation is "bad" now because shouters want to be able to get the most Plasm per trip as possible. It is no different than trying to get the most kills per set of playtime for Voidwatch or the most EXP per hour when Meriting on Colbri.
This isn't selfishness, it's business. The shouter knows that he or she isn't trying to get the most EXP/Plasm/Kill/Gil per hour for his or her own selfish needs (not always), he/she is trying to get the most for the hardest to fill jobs: support (and now PLDs). Support and mage jobs are not always the most entertaining jobs, they require the most attention to play, and require higher than average reaction to play. DDs do not necessarily require such high requirements: they don't need to look at other player's health bars, they don't need pay attention to TP moves that give status affects to other players, and they don't need to keep the group buffed almost 100% of the time. Most of the DD's job duty is on auto-attack. The differences alone make the DD jobs more appealing and mage jobs unappealing to most players. It's obvious when we see the large number of DDs compared to the number of support.
We have an oversupply of DD and an under-supply of support. So, in order for the shouter to get best support he or she can, he or she is going to need to appeal to them. Since there is no way to measure the skill of a player without knowing the player themselves, the easiest way to measure "skill" will be requiring the DDs to have RMED. This restricts the number of DDs, but potentially increases the number of currency/exp/kills per trip/pop/run. This appeals to the under-supplied support jobs.
This is called "supply-side economics". It is working here.
Now to stop the issue that is Hawklaser brought up about progressive credentialism (Quote: "As the DD situation is only going to get worse on gear requirements as things go on, and help for older content can be very hard to get at times.") that is created because of supply-side economics of our job economy, players will need to do five things:
1. Play a support job
2. Start their own shout group and they can set their own criteria
3. Join a LS that doesn't require RMED for events.
4. Try to convince the support job players to reduce their currency/exp/kill per hour for the same amount of work as he or she would be doing if he or she is being "paid" more per trip/pop/run.
5. Ask SE to make AH equivalents (as Hawklaser had suggested)
#1 and #2 are doable. #3 might me harder. Good luck with #4. #5 could happen.