Disagree. The way Apple does it is retarded. On my Android phone or tablet, I can plug in my 3TB USB hard drive and drag and drop files to and from the phone like I can on a PC. It's handy, not having to go through stupid iTunes for everything.
Both devices (iPad and Surface) are successful because they address different groups of "others". No company like Apple or Microsoft will maken something that is tailored to only 1 person.
You're also incorrect about having to go through iTunes. Both GoodReader, iBooks, Adobe Reader (or whatever they call it now) and many other apps have wifi sharing or are able to take use of something that currently is a big thing: cloud computing (or more specific, cloud storage). This can either be a third party cloud service from Apple, Microsoft, Dropbox, etc. or something you control. Also, NAS is something that is very very popular due to people having more than 1 device. Using a NAS vs an external USB drive has a lot of advantages like not having to plug it in/out and moving it around. Not to mention the manufacturers that come out with drives sporting a Lightning connector to connect the storage to an iOS device.
Like I want PDF copies of shop manuals for my motorcycles and ATV on my iPad. Except that I have to keep multiple copies on the iPad because it only wants to open with t he default app. But the default app can't open a file larger than 50mb, so I have to try and open it with the default app, it fails and tells me to pick another app, so I do, and the thing isn't smart enough to remember that next time... I have to keep telling to use the other app, but only after it forces me to try the app that it already knows cannot open it.
Then why isn't this a problem for other people? Being able to read PDF documents is the main reason why the iPad is such a success with governmental organisations and businesses all over the planet. Keeping many copies is also your own choice. You could also choose to simply use only 1 app and store all the info there.
So much easier in Android when you open it the first time and it simply asks you, "here are the apps that can open this, which one do you want to use from now on?" and it remembers it.
That's the only big thing iOS misses. Being able to set non-Apple apps as the default. Android and Windows are able to do this. However, Apple did ease the pain by allowing apps to interact and share data.
With a real file system, I can keep a folder of "My Stuff" and click on it like a PC and just open things up, rather than have to load and keep copies within each app that I could use, and have to know and select ahead of time.
iOS has a real file system, every computer has

Nobody wants to fiddle directly with the filesystem, we want to fiddle with files. That's a completely different way of working. Basically all Windows Explorer and the Android apps are: just apps where you can manage files. There are apps like that on iOS and lots of things you can do in Windows Explorer can be done in those apps. The only thing that is a bit more limited is how you share data among apps. You generally have to open the app containing the data and then share from there whereas in Windows/Android you can open the app where you want to use the data and access it. This changes when you use cloud storage like OneDrive, Dropbox or iCloud Drive since lots of apps have support for these services (this also applies to Windows and Android).
This is "user friendly"? LOL
Yep for many it is user friendly because it takes way an awful lot of complexity that is a desktop computer. For people who require/want that complexity it obviously isn't.
Apple iPad sales are plummeting, while Android tablets like the Galaxy are increasing.
Tablet sales are plummeting be it iPad, be it Galaxy. Does it say something? No because there is no explanation as to why it is plummeting. Could be that people have enough with tablets in general, can do much longer with tablets, don't have money to buy new ones (economic crisis is still going), don't like iOS, are waiting for the iPad Pro (like many on the forums here) and so on. It might be a short term thing as well. Things could change in a couple of months.
I don't know where you are getting your competitive intel from, but you need to update it from 2012...
You need to update from 2002. Tablets are not advanced devices like tablet pc's back then which is why they are such a success right now (and why tablet pc's back then failed miserably). We're talking about tablets here, not tablet pc's!
My buddy is an architect and designs new homes and he does it on his Surface 3 Pro... An iPad could never handle that...
My buddy is a truck driver and transports a lot of cargo and he does it with his huge truck. A bicycle could never handle that...
Or in other words: know the products before you compare. The iPad is a simple tablet designed to do simple tasks like reading ebooks, browsing the web, watching videos, email and so on. It isn't designed to do boatloads of programming, 3D designing, photo editing, CAD/CAM and so on. The Surface Pro can do those tasks better but isn't designed for it either. You don't want to run any software like SolidEdge on what is in reality just an ultrabook. SolidEdge requires more from the GPU than the GPU in ultrabooks can deliver. If you want to do something like that you're gonna be better off with something like the HP Zbook with an ISV certified GPU.
The file system is annoying, have each app locked in a sandbox...
To you yes but it is a requirement for a lot of others. This and other security features are the main reason why the iPad is doing so well as a business tablet. Android doesn't fare so well in that area. Windows tablets are doing ok too, mostly because it is an ultrabook in a tablet form factor. Both Windows and iOS have integration with business tools that allow you to lock it down.
The main reason why Android has high figures is quite simple: they are on a lot of devices ranging from very very cheap to a bit expensive. Lots of those devices are used in promotions. Microsoft has made devices that are cheaper and it shows. They completely overtook Android in countries like India. One should ask the question how on earth the pricier iPads sell so well. Something else about the iPad is making people want and buy one.