There is no mechanism currently on the AppStore for doing a SaaS strategy for Apps. In app purchase is set up for one time purchases, not for recurring subscriptions. This is a brand new feature, and it dovetails right in with the SaaS business model.
Apple doesn't take %30 of "ALL subscription revenue" though people seem to keep saying this because it plays into the "apple is taxing us arbitrarily" meme.
IF I wanted to, I could set up a webpage with paypal, and charge people directly, and save %30, minus paypals fees. However, paypal has proven itself to not be a trustworthy company in the past so there's an additional cost there, but that involves a lot of setting up servers, dealing with customers, etc. There really is a lot of effort required to sell things directly to the customer, there are liability issues, and if I were to outsource all that work I'd be buying several services that ultimately would take around the same total cut.... and I'd be more stressed on top of that.
All that stress and time is energy that would be better spent on additional features or products. In fact, it would produce much higher returns because it is more highly leveraged. The effort that requires on the current app is just maintaining the status quo (I think there are zero customers who would rather pay by paypal than iTunes).
I find it interesting that you say I'm leaving "a huge amount of the fruit of [my] labor on the table", as Apple is freeing up a huge amount of time and energy to improve the app or do new products.
Look at the alternatives- I could give the app away free on android and let google advertise on it. How much would I be giving up in "google tax?" %60. And they only revealed that it was %60 after Apple made it public that they give developers %70. Who knows what it has been for the past decade before google had to be public about it. Plus, I know how many copies of my app are sold, I don't know how many clicks google disallows. Sure there are competing models, or I could do paypal with an android app (this would at least have the advantage of selling into all the countries paypal supports but not google checkout.) but again, that is time I have better use for.
OR Amazon which will make me give up price control and possibly %80 of my revenue. These terms are not "just as bad" they are far far worse.
And finally, before this, I was paying %30 on the initial sale in the Appstore. Nobody seemed to be complaining when Apple originally introduced that model-- and for good reason-- before that Carriers controlled app sales on phones and were much worse than publishers, taking %90 in many cases.
Apple has greatly reduced the friction, and has added a new feature that lets me offer a better, more profitable model while simultaneously capturing many more customers.
It's a no brainer.