I think that 50 $/€ per year is normal and nothing new. If you use PDFPen Pro on your Mac, you will have to shell out the same amount of money per year to stay current. Yes, it is no subscription, but still…
What is new is that PDF Expert 7 is the first PDF app on iOS I am aware of that is almost as feature rich as a MacOS app. What I do not understand is the omission of OCR in PDF Expert 7 because Readdle sells Scanner Pro and this app has OCR.
I would be fine to pay 50 $/€ per year for an app that enables me to do anything in a PDF both on my iPad and on my Mac. Maybe, with SwiftUI this will become feasible eventually because of having the opportunity to have a unified codebase to some degree.
Until this happens, I will keep using PDF apps on iOS more as a reader and for annotations and I will create and edit PDFs on my Mac. Otherwise I would have to 100 $/€ per year for PDF editing both on my Mac and on iOS. And that is too much.
I too do not see subscription models as an issue, but at the same time I do not see myself jumping onto every subscription that is available and interesting.