Disclaimer: I am not a designer, so this is just my educated guess and observations as a long-time (February 2003) digital scrapper.
I definitely think there is 'too much' digital product on the market in that there are too many designers who release so much product that prices, generally speaking, within the industry are lower than they should be in order for designers to make enough income to support themselves/their families/make more than minimum wage. Prices have increased in the last several years, but kits/collections have also expanded as well. However, if a designer were to dramatically increase the prices on any of her kits by more than a dollar or so, she would price herself out of the market altogether as customers would buy similar product at a lower price from other designers. While I see digital scrapbooking customers as some of the most loyal in the crafting community (in part because the digital community is individuals who we get to know on a personal level via their blogs, layouts, etc.), I also think many customers would not spend $15 on a very well done birthday kit by a well-known designer when they could spend $10 on a good birthday kit by an up-and-coming designer, KWIM?
With that said, I think regularly retiring product is a good way for a designer to generate some income off a previously released and no longer being purchased frequently item in her shop. I don't know for sure since I am not a designer, but I believe that most designers make the bulk of the income they are going to make from a product during its first week of release (with the exception of very specific sports, hobby, etc. kits). This is why most designers have a creative team to whom they release their product ahead of time in exchange for sample layouts to use in their store listing and posted around Digi Land with direct links (even though I think CTs have lost a bit of their effectiveness with the advent of social media and scrappers moving from site message boards to Facebook groups). Once that initial week has passed (and the sale/discount many designers offer if you purchase something the first week it's released), I believe purchases drop off fairly quickly especially after the season passes when that kit would be current e.g. after December 25th for Christmas kits. When that season comes around again, I would think you would have some new purchases for a particular kit, especially if it were for something specific within a season like an Elf on the Shelf collection, but after the second season passes, I would think there would be too much new, but similar, product to get many more purchases out of that kit.
Retiring that product, however, may generate some 'last' income off that product as customers who were previously on the fence or had wishlisted something may finally take the plunge and buy something knowing that it will no longer be available to buy in the future (and with a good enough discount on the item).
There is no good way to receive any use or resale from a kit you have purchased since you have not actually purchased the kit but a license to use it. To me, it is similar to Netflix, Hulu+, and other subscription services I belong to. There are many movies and shows that I have no interest in viewing on those sites, but I pay to use the service because there are more offerings that I am interested in consuming than not. I don't own that media, either, but pay to use it in the same way that I pay to use digital kits.