You're leaving out the crucial point that the issue of ROI was initially brought up with regard to the matter of "printing too many crunchy books too fast is unsustainable and unprofitable," specifically in that it was neither of those things, and that the only reason WotC moved away from doing so was due to ROI.
I'm going to have to strenuously disagree here.
Let's start by making the obvious point-
Let us assume, for purposes of argument, that one of the two things is correct...
1. More material will always result in more revenue over the long term. (Ignoring ROI).
2. Less material will always result in more revenue over the long term. (Ignoring ROI).
If either of these were true, then logically the correct answer would be for WoTC to either (1) print an infinite amount of material; or (2) print absolutely nothing.
Now, we know that these extremes aren't true. It's almost like there are these curves ... call them supply and demand ... that work to influence the market. But, just as importantly, there are different business strategies as well.
Take, for example, the Birkin Bag. Hermes could, quite easily, hire more people and pump out more Birkin Bags. And while that would undoubtedly create more revenue in the short run,
the entire point of the Birkin Bag is that it is exclusive and hard-to-buy. Pursuing a different business strategy would lead not only to the death of the Birkin Bag, but also the diminution of Hermes as a brand. We all learned this basic economic concept when we were in kindergarten, although the example used involved more geese and golden eggs.
Now, the RPG book market isn't the same as the Birkin Bag market .... obviously. Value isn't created through enforced scarcity. Still, there is something to be said for a reasonable level of output; this has several advantages- first, you are not overly taxing your consumers' spending. Second, you will not have so many undifferentiated products out that new consumers may be turned off. Third, you are less likely to overwhelm your consumers when they look at their bookshelves. A moderate pace with gradual releases is not just "ROI," it's also an investment in the overall health of the game.
But what about Paizo? Well, different companies can pursue different strategies. It is certainly relevant that Paizo has a large number of subscribers- a model that WoTC does not use for physical media. More importantly, I think I can safely assume that most PF players are considered more "hardcore," whereas WoTC is marketing to a much more diverse, and casual audience. To put it even more simply- different companies are pursuing different strategies for different consumers and to protect different brands.
In saying all of this, I do not mean to say that WoTC's release schedule is "just right," like some type of ersatz Goldilocks analysis. It has been successful. Could it be more successful if they released more books? Maybe. But maybe it could be more successful with fewer, better titles! I would say that the rest of this is noise- they decided on a slow and steady release schedule, and that has been, to date, a reasonably prudent decision.