Sorry, iOS is a no-go for anything CLOSE to serious work:
- No filesystem so you can organize, track, archive, and backup your project data the way YOU want it;
Wrong. iOS offers a file structure and access to your Mac fold hat ers and folders on iCloud.
- No ability to add software other than through the walled garden;
There's a very good reason for this. . . no easy way to ad malware. But there are over 2 million apps on the Apple iOS App Store that can provide everything you need. If you are a corporate users, Apps can be side-loaded through your corporate access to your Corporate owned iOS devices.
- iOS device are modeled after a reader / casual checklist at best user interaction model;
Obviously you've never really used one. There are productivity tools for iOS devices and many people have used them for professional productivity. iOS devices have been used to even produce and film full length feature films, write novels, and do post-production work.
- not suitable for hours-at-a-time work;
See above. . . again, you have not bothered to try doing what you say is not possible.
- screensize is very limited;
An iPad/iPhone can drive any size screen you want to name. . . up to 4k, 5k, or 8K screens. That is not limited.
All you've done is show you've fallen into the hype of negative information about iOS.
I would not use an iOS device to program, develop databases, etc. . . but they are capable of doing so.
Macs will still run macOS, not iOS. macOS is fully capable of running on Apple’s A-series processors. I expect the transition to start by 2020 at the latest for many reasons, not least of which Intel technology is lagging.