There is NO reason why OSX couldn't or SHOULDN'T be able to run iOS Apps. There's a lot of games, for example, that would be nice to be able to play on a larger screen and with a trackpad or mouse input that the developer has not ported (for whatever reason from doesn't care to laziness) to OSX. One could simply have an iOS Mode icon on the dock for such a thing. Developers already have access to an iOS emulator. And yes, since iOS software is for ARM chips, they're not going to run hyper fast, but they can still run well enough under just-in-time emulation.
I'm not saying that OSX itself should be more like iOS, but it could encompass the software quite readily and with the addition of touch-capable screens, you could even have the option to run the software with the same touch inputs. Now whether you'd WANT to is another matter, but I see no reason what-so-ever to not give the OPTION of doing so if you have a need or desire for it for whatever reason. Given the idea of an iPad Pro, it seems all the more reason to have more options and merger for Apps that could easily move from mobile to the home without having to run two "compatible" Apps. Just run the iPad Pro App on you Mac directly. Make such iOS Apps more "Mac Aware" (i.e. designed for mouse/trackpad input alternatives so that when run on such a device or with a bluetooth version with the iPad Pro, you have the option of doing so. I see nothing wrong with that at all because it's expanding the iOS paradigm into the Mac but doesn't mess with the Mac itself (i.e. Mac only software).
Now whether iOS devices should be more OSX-like is another question altogether, but certainly having support for Bluetooth mouse/trackpad input especially on some iPad "Pro" model that's rumored certainly makes sense and SD Card and/or USB inputs for some Apps makes sense as well at some point. I can even see validity for a crossover iOS device like a "Pro" iPad that acts like an iPad (and runs iPad software) as a touch-screen device, but could also have a kickstand type dock mode that with the addition of a full keyboard/mouse could run full OSX or at least something closer to it. And there is where the above OSX proper "iOS Mode" could create the "hybrid" iPad "Pro" that actually runs OSX but has an iOS Mode as a tablet device. Those that could use such a hybrid would find it more useful and those that just want a giant-arse iPad would leave it in iOS mode. And those of us that would like to play an iOS game on our Mac would just activate the dock icon to do so. But the point is that you wouldn't "merge" iOS and OSX, but simply make OSX iOS capable and some devices like an iPad Pro or even a flip-around Mac Air could operate as EITHER one. Some apps could then obviously operate in either mode.
As long as the Mac experience itself doesn't change, I don't have an issue with OSX getting iOS modes/features. Where Microsoft has gone horribly wrong is trying to force their mobile "Metro" GUI as the DEFAULT. If they simply had a Mobile/Desktop "switch" that remembers the last setting or a default and switched between Metro and Windows7 interfaces, I don't think they would have gotten any real flak. You use it in whatever mode you want and everyone is happy. You make the START button go to Metro and you get complaints (except from the people making 3rd party add-ons that turns it back into Windows7 with a newer DirectX). Of course, Microsoft's "Surface" products are just cheap'n'crappy. That wouldn't be the case with the Mac if it's done properly.