Collecting sold out single volumes into compilations is a standard operating procedure in the comics & books (and to a lesser degree in the rpg) industry.
I don't think comparing the comic model to Paizo's model is necessarily a good analogy for your reasoning here.
Bear in mind that, in the comics industry, advertising pays for a substantial part of the cost of the printed comic, which is why monthly comics are still being created (as opposed to going over full throttle to trade paperbacks). That isn't the case (at least to the best of my knowledge) for APs.
Additionally, comic publishers don't bother waiting for singles to sell out before collecting them in trade (just look at all the single issues that fill the dollar bins in any comic store. Volumes of them these days, as opposed to once upon a time). So you end up in a situation where the retailer- who orders his issues on spec, hoping for his retail customers to buy up the singles- now has a lot of singles left to try and dispose of at a much lower profit margin. (Which has hurt a lot of retailers substantially.)
If anything, I would think that the comic model would be a good cautionary tale to Paizo [u]not[/u] to collect APs on any kind of regular basis. I would assume that they retain the bulk of their print run, with only a small percentage of it going out to retailers to sell, meaning that they would be the ones with the largest volume of what are presumably less appealing (to purchasers) single issues when they do their collected "reprint."
(Just as an aside, I'm kind of glad they haven't put out collected volumes of more APs than they have. I really love the collected/revised RotRL, but I was somewhat disappointed in CoCT. I felt that the layout was not up to the quality of the layout in the RotRL collection, and did not care for most of the changes. I prefer the single issues, even if they do require conversion to PF.)