It's buying into a promise that your monthly payments are going toward continuous improvement of the product – a promise that might be kept, or might not – and even if it is, for how long? If updates stall out after awhile, or if the new features don't solve anything for you, it's not like you can choose to skip an upgrade. And the more you use the product, the higher the barrier to switching away. I agree, Craft looks lovely, but there's no way I'm locking myself into its business model.
I say this every time, but this is a solved problem. Developers of software like Sketch and Bitwig Studio have the right kind of subscription model, where you subscribe to updates, not access. If you don't care for the latest version, you can let your subscription lapse and keep using the last version you're licensed for. Apple has all the pieces necessary to simply gate App Store updates via subscription, but they apparently have no motivation to do so.