I think the subscription model is the right choice for Final Cut Pro for iPad because...:
- It is a completely different software from Final Cut Pro for Mac: the UI and way you interact with the software is completely different, which means Apple had to two spend a lot of money re-building Final Cut Pro for the iPad.
- The intention is most likely that you'll do most of your professional editing on the Mac, and only use Final Cut Pro for iPad when you're traveling (or don't have access to your Mac). Most people don't want to re-spend that much money on a perpetual license again for something they'll just use as a backup plan.
- Final Cut Pro for iPad is super cheap compared to the competitors solutions.
- In order for Final Cut Pro to become more relevant again Apple needs to find a way to generate more money from it (so they can invest more in new features). I purchased my license 10+ years ago, and have gotten fantastic free updates ever since, but never payed a single cent for Final Cut Pro since I purchased it.
I understand your frustration, but considering the above I think it's understandable why Final Cut Pro for iPad is a separate software. What I will say, though, that makes me frustrated about this is that Apple doesn't allow third party apps to have separate pricing and versions between different OS, so it's not fair that they bend their own rules in their favor.