Get back to me when someone starts replacing "legacy apps". I am not down on ios apps, but nothing on ios will be competing with "pro apps"...er..."legacy apps"...anytime soon. Remember, just 2 years ago the desktop was legacy! And even these apps were released on ios--who wants to be doing work in a complicated video compositor/editor or sound daw without a mouse (or trackpad). No thanks! Love the IPP--its just gimped in certain ways.
The larger question is hybridization. The hybrid form factor will continue to shrink. Whether MS can make windows more mobile like is an open question. I personally am surprised how little progress has been made
Once upon a time I needed a desktop computer for creating PDF forms. Then PDF Office came out, which is not only more user friendly than the software I used on desktop, but also faster (/has less issues with large PDF files than an i5-4670k PC) AND has some nifty automation that the desktop software lacks.
In the last year, OCR seems to be the part of my workflow that is just now becoming more common on iOS.
To name a few examples of actual legacy software made redundant by iOS apps in the last couple of years. I think you have a notion in your head that it's the software
you use that qualifies as this type of app.
I see this request a lot and I'm really wondering what the advantage of this Mac base would be as opposed to having a MacBook and an iPad. Is it to save money? Considering you'd only be losing the cost of the display of the MacBook, and considering Apple would have to charge more money for the new design and manufacturing of this more complicated detachable system, I think you would probably break even. Best case scenario at MOST you might save a couple hundred, but definitely not the cost of a whole device. You're still buying two devices, only one of them is gimped because without your iPad it's a paper weight. Even assuming your iPad is ALWAYS with you, having to pull it out and attach it every time you want to do a simple quick Mac task is not exactly the kind of user experience that I think Apple is looking to give. Why not just spend a couple hundred more and have an actual functional MacBook that doesn't always require attaching an iPad, which will probably make for a clunky laptop experience anyway? I think that would be Apple's perspective.
1: Total package would likely be smaller and lighter, unless you're comparing to bringing a Macbook.
2: Shared resources. USB C charging, shared battery(!), USB ports, display out, access to each other's files and other continuity features.
3: If done correctly, there's no reason the iPad couldn't have the base's display as an app inside iOS. Split view macOS/iOS, copy/paste between, etc.
4: macOS app with Apple Pencil. Nuff said.
5: Dock base to external monitor and it's not dependent on its iPad counterpart.
and probably a lot more I'm forgetting atm.