But the Blights have, by at least number five, been shown to be very poor ways of doing this; each seems to fall more quickly than the last, with the archdemons being presumably permanently killed. One would think that if the Old Gods were deliberately attracting the darkspawn to them, they could just stop now.
They're falling faster because Thedas has gotten better at stopping them. The length and damage of the First was a result of the search of how to kill one, but that was the largest obstacle.
I don't see any particular reason why the Old Gods are deliberately attracting the darkspawn to them either: it could be an involuntary consequence of their imprisonment. They aren't even necessarily 'awake': the Old Gods contacted the Magisters through dreams in the fade, not through simple telepathy. They could, quite literally, be snoring through the Stone.
Edit for addition:
The point I'm making is that the Old Gods dying off via the Blights doesn't really imply the Darkspawn weren't intentional. The creation can be intentional while the continuation is consequence that they couldn't stop if they tried.
If there's anything to suggest that the Old Gods aren't under their own control once corrupted, it's the fact that they immediately start on the warpath rather than bide time to organize, build the hoard, and most of all find and corrupt another Old God for redundancy. The effect of the Taint could well drive them to the Blight even if it isn't the strategically best course of action. There's a degree of impatience there.
Which says nothing about what their original intent was, mind you- just that it went horribly right.
On an entirely unrelated note, because it would seem the Blights could continue forever if the ArchDemons just flew in the air and never joined the battle, who else has wondered if the gryphon extinction might have been an unnatural occurrence?