As long as the angry mob is out anyway, let's break out the guillotine and look at this question with the motto of the French Revolution.
Liberté, égalité, fraternité - Liberty, Equality, Brotherhood
Under a subscription MMO model, customers are relatively equal in value. Longtime subscribers are going to pay more in the long-run, and may serve as pillars of the community in a way that retains more than just their $15. However, when it comes to the quarterly earnings call, each customer's $15 is the same.
If anything, this equality motivates companies to focus on endgame. The raider is most likely to quit now if they are out of content, and most likely to be retain-able if the studio makes more raids. The solo player may also quit when they run out of content, but they might re-roll instead. Worse, the developers might spend their effort on more solo content only to have the solo player beat that content as well and quit anyway.
The non-subscription model adds variety to the payment models, and, in so doing, adds Liberty. A non-subscriber might pay less than $15, while non-subscribers and subscribers alike can potentially choose to pay MORE than $15. This freedom means some customers are literally worth more than others.
The obvious and most-feared extreme is that the one person who is addicted to gambling for cosmetic items through so called "lock-boxes" is literally worth more than a small guild of loyal players who had been with the game in its previous incarnation. That aside, having a model where players can pick and choose what they pay for potentially reverses the developer's incentives for the future direction of the game.
If only a small proportion of players raid - and said players vehemently oppose any mechanism whereby they make themselves proportionally more valuable by paying more money, on the grounds that this would be "pay to win" - then only a small proportion of future development can support them. By contrast, if the majority of paying customers are located in the leveling curve, that is where the developer must focus their efforts, even if said customers are certain to depart after spending some amount of time in game.
To use another concrete example, the patch will add a new NPC companion, an HK-51 droid. Bioware hopes that nonsubscribers will pay for an unlock to access the content that awards this NPC. However, even though companions are basically solo tools that cannot be used in serious group content, the questline requires a max level character and several group dungeons. Merits of this decision aside, a non-subscription game can ill afford to put barriers between customers and stuff they want to pay for.
With this kind of split in the interests of the playerbase, it should come as no surprise that Brotherhood is in short supply indeed.