We'll see OS X or a redesigned iOS in iPad Pro gen 2 or 3, but no later. And it will take a lot of redesigning for iOS to be able to become Apple's new answer to Windows.
Apple's answer to Windows will be exactly like how it answered everyone's request for an Apple netbook, not by jumping on the bandwagon but by ignoring it altogether and releasing its own unique take on that problem (which in that case was the iPad).
I personally find Windows 10 to be an abomination in every sense of the word. 2 clearly differing OSes with different modes of input bolted together. If Windows 10 somehow feels "right", that's only because it's still a desktop OS at heart, being used by people like you and me who grew up accustomed to legacy computing standards. We are not the future. We are the past clinging on to an outmoded form of computing because that is what we are used to and what has worked for us, and have precious little right to impress our outdated standards on the next generation of users.
As such, I don't believe for one moment that Windows 10 represents the future. That said, it is still the present, and will at best serve as a stop-gap measure for Microsoft for the moment.
Apple's answer is to steadily improve on the capabilities of iOS until the iPad (running iOS, not OSX) is as capable as a PC in its own right. I believe this to be the right way moving forward, because more and more people are growing up with mobile devices as their main computing device. Don't be surprised if and when these people graduate and enter the workforce more accustomed to getting things done from a tablet than a PC, and when they do, Apple will be there waiting for them with just the right device for them in tow.
I and a lot of people would also be happy with a PadOS with freeform resizing and multitasking of apps. "One size iOS" does *not* fit all.
I think the number of people who share your sentiments is a lot fewer than you make it out to be. And this number will further dwindle once they get their wish, get freeform-resizing apps on a tablet, and realise just what a horrid experience it really is.
Something like that already existed in the form of Windows 7 tablet PCs. They never caught on, for the simple reason that the user experience sucked. This is one of those things which sounds good on paper, but sucks in actual usage.
The whole point of iOS is precisely so that we could do away with archaic PC legacies like multiple overlapping resizable windows, which were honestly a pain to deal with. Do what you are suggesting would be a legitimate regression.
But it's still an equally logical step that the entire OS X will come to the iPad Pro soon. The people who only want iOS launching will stay permanently in LaunchPad to open fullscreen apps. Everyone else can connect a keyboard and mouse to use it as their desktop/laptop at home with full multitasking and deep desktop apps, and then bring it with them as a touch device when they go out. That is anything but ridiculous.
What's ridiculous is your belief that Apple will do something so anathema to their design tenets. You are referring to a device running 2 separate OSes, supporting 2 different types of software and 5 varied forms of input. Apple believes in simplicity, which means getting things done with as few resources as possible. This is why the Macbook Air has so few ports (to make it thinner and lighter) and has so good a keyboard and trackpad so users can get meaningful work done without feeling like they have to plug in a mouse just to drag-select stuff properly. The last thing Apple is going to do is bolt everything together to create some sort of frankenstein hybrid device.
Minority Report. Placing multiple rectangles on the screen and resizing them freely and stacking them like Tetris. Running a small 1 inch view into some stock ticker app while an email client takes up most of the screen, and some web browser is displaying a website, and a Skype client sits in the corner, etc. Xcode has supported elastic layouts and it is the layout mode Apple encourages. So most apps are ready for this.
You can already sort of do this on an iPad. Split-screen email and your browser, and run Skype in PIP mode.
And yes, we will get either true non-kiddified iOS multitasking or the whole OS X (with iOS app launcher built-in, obviously). Bookmark this thread and come back to it in Oct 2016 or Oct 2017. You'll see. Apple cannot aim at kids and moms forever, unless they want to give the desktop/pro market to Microsoft... actually, I hope to hell they don't lose their roots as a computer company. But toys sell more units, so yeah there is reason to be worried.
We shall see indeed. Let me make a bold prediction here and now - You lose.
Well, guess what? The majority of workers out there are not computer engineers and programmers like yourself. The majority of PC-using people are these kids and moms whom you so deride. It's easy to do more with more (complexity), but I also feel it is irresponsible to throw this complexity back at the end user and place the onus of figuring all this crap out on them.
I firmly believe that while desktop PCs will not go away, they will increasingly take a back seat to more mobile forms of computing, which is precisely what Apple is betting on. You will still have your Mac Pros and your Intel Xeon workstations hooked up with triple 4k displays for stuff like video editing, but that is done by less than 1% of the world's population?
The real challenge is in doing more with less (complexity). That's what iOS is aimed at - making computing more accessible and convenient to the majority of users who don't necessarily need a full-powered desktop OS, multiple screens or a render farm to get things done. And I repeat, you do that by continuing to build on the foundations of iOS while keeping its core strengths (security, ease of use, battery life), not by abandoning it.
Ceding the desktop market to Microsoft (which likely won't happen; Apple will definitely continue to make and sell Macs) will be like the equivalent of ceding the low-end, low-margin smartphone market to Android OEMs. Ignore meaningless titles, all while keeping your eyes on the real prize.
Desktop apps don't magically become "fat". If the features match in both apps, and they use the same algorithms and same graphics assets, then they will use exactly the same amount of RAM on both iOS and OS X (internally; not counting OS frameworks, but those are usually shared among all apps).
I like what Adobe is currently doing with its Photoshop app for iOS - breaking up the core app (which I always felt to be overly bloated and needlessly complex) into multiple smaller and lighter apps, each of which is optimised for a singular purpose. This in turn makes the apps more easily accessible to users. I think this is the way forward for iOS.