To be honest, not giving at least a PDF of the extra material to all the pledgers above a certain minimum level strikes me as kind of chintzy.
Giving away PDFs reduces the perceived value of PDFs. Here's an example:
A 144-page hardcover like
Slaves of the Machine God is $40, it's PDF is $15.
The lowest pledge level on the AotA campaign is $20 (for just the PDF of AotA).
The next one is $50 (for the print + PDF of AotA).
The next is $60 for all the PDFs.
And then $160 for all the print books. (To simplify things, I skipped the retailer-only level and the level that includes another book already in production.)
If a stretch goal unlocks a new 144-page book, adding the $15 PDF of that automatically to every backer level is nice for the $20 and $50 backers. But it doesn't help the $60 or higher backers, so there's little incentive for someone to pledge at any of the higher levels because they're not getting a greater benefit than anyone who pledged at the lowest levels.
I feel like calling them "stretch goals" in that case is misleading.
I think you're mistaken about what "stretch goal" means. A stretch goal in a Kickstarter (as defined on the Kickstarter site) is "a funding target set by the project creator beyond the original Kickstarter goal." That's all it is. It doesn't mean that one or more backer levels or rewards are suddenly worth more. A project could have a stretch goal where the creators add backer names to a book as a thank-you, or that they'll give away X copies to libraries/schools/gaming clubs, or that they'll make a thank-you video on YouTube; none of those make any of the backer levels better.
That said, the stretch goals in the AotA Kickstarter do add value--different stretch goals affect different backer levels in different ways. Which can be an incentive to switch to a different backer level.
And our Kickstarters always end up with those "all the X" backer levels being a really good deal. For example, the Numenera Discovery and Destiny Kickstarer had a $100 "all the PDFs" backer level that people weren't interested in because the project launched with just two big books ($20 each). By the end of the Kickstarter, TEN additional items were unlocked via stretched goals, and the total value of all of those PDFs ended up at about $150 (just me estimating, as not all of these are available yet so I don't know the exact PDF prices of upcoming releases). Which is why in the last few days of our Kickstarters, a lot of backers increase their pledge to the "all the X" backer level.
At the $60 level you can't get a physical book.
The $60 "all the PDFs level" isn't a backer level for people who want physical books. We have a lot of people who only want PDFs, or only want print books. Two different types of customers, two different kinds of backer levels. It sounds like the PDF-only backer level isn't something you're interested in, and that's fine.
If you want a physical book, you have to pledge at least $160 to get ANY stretch goals, which frankly seems excessive to me.
I am not a huge Kickstarter devotee, but I have backed a fair few over the last 5-6 years, and this is the first time I have ever seen one that set such a high threshhold to receive stretch goals.
The $50 pledge level (which gets you the AotA book in print, and the PDF, too) has already been improved by the $135k stretch goal, which expanded the book (more pages) to include another full-length adventure.
And the $20 PDF-only backer level was likewise improved by that very same $135k stretch goal.
Yes, the next stretch goal (for the deluxe book) doesn't improve either of those backer levels, it adds to the higher backer levels. But remember that not every stretch goal has to add value to all stretch goals. Different stretch goals are built for different customers, and not all incentives (which is what a stretch goal is) are meant to entice all customers. I go to Starbucks every day to get a drink, and a one-day promotion to try their bacon breakfast sandwich is useless to me (I'm a vegetarian), but there will be other one-day promotions that are tailored for my interests.