While I can't read the minds of the publishers, I can look at the actual sales data, draw some pretty graphs, and find out what's actually been happening to the sales figures in terms of cold, hard numbers instead of guesswork and speculation. :-)
All data comes from the ICv2 website.
The graph shows sales of Dark Horse's Buffy the Vampire Slayer and IDW's Angel comics, to the same scale. I've included the two non-sequential 'Buffy' comics, Tales of the Vampires: The Thrill and Willow: Goddesses and Monsters, because they are in direct continuity with Season 8 and were published in gaps between the regular comics. I haven't included IDW's series like Spike: After The Fall or Angel: Only Human because they're spin-offs published simultaneously with their main series.
The graph confirms that the comic sales have, indeed, been dropping steadily. For the 'Buffy' comic there was a big fall-off in the first few months, then a slow but steady decline each month thereafter. The big dip and recovery visible in the graph was for the The Thrill one-shot. However, what the graph also shows is that IDW's 'Angel' series has followed an almost identical pattern: a steep drop after the opening issue followed by a slower but steadier decline thereafter. Their curve isn't quite as steep as the Dark Horse one, however; perhaps suggesting that 'Angel' readers are a little bit more loyal - or conversely, the 'Buffy' comic initially attracted far more non-comics readers who tried a few issues, didn't enjoy it, and dropped away without ever even bothering with IDW's comic.
How do these figures compare to the overall market? This graph shows the same data as above, but I've also plotted the sales figures for DC's Batman comic over the same period of time:
Now, the Batman comic was first published in May 1940, is currently up to issue number 695, and is still regularly in the top ten bestsellers each month. The ICv2 website even uses it as an index to judge sales of all other comics against. What the graph shows, then, is that for the last two years at least, the decline in 'Buffy' and 'Angel' sales has been matched at pretty much the exact same rate by 'Batman' sales. The difference is that DC seem to have mastered the secret of re-booting the series every year or so, and adding on an extra 40,000 or so readers who are then gradually lost again over the course of the year.
Looking at the 'Buffy' numbers in more detail, here's how the sales figures break down over the course of each arc:
As we see, the initial drop-off continued through the first two arcs. However, by issue 9 and the two Joss-written stand-alones that followed it, the sales figures were pretty much stable. There was even a slight up-tick in sales at the start of each of the next two arcs, presumably as new readers picked up the comic to see what was happening. However, the 'Fray' arc was less good at retaining readers than the Japan story, and then 'Predators and Prey' started shedding them in large numbers. To be fair, this was also the time that all other comics in the market started seeing sales drop, but this was also the arc that drew most fan criticism. 'Retreat' seems to have slowed the decline, even if it couldn't stop it; and the curve seems to be levelling out again with the most recent issues. We don't yet have the sales figures from 'Twilight', though, which will be interesting...
Finally, here is a comparable graph for IDW's comic:
Curiously, while the overall curve of Season 8 sales has been a steep initial drop which then bottoms out, the 'Angel' sales are almost the opposite. There was an initial dip after the first two issues, true, but then sales were pretty much stable for the first eight or nine issues of After The Fall. However, while the 'Buffy' comic's sales were stabilising at that point, the 'Angel' comic sales instead began a much steeper decline. This continued right through to the end of After The Fall and beyond into the much-criticised Aftermath arc. It was not until Brian Lynch teamed up with Juliet Landau to write the Drusilla two-parter that their sales recovered a little. While IDW's sale figures are still trending downwards, they do seem to have finally stemmed the steepness of that fall. How the revelation of Angel's role in Season 8, and the associated controversy, will affect their figures is still to be seen.