The difference between userbases is that not great currently. It's 500 million Android devices to 400 million iOS devices. The iPhones had a great head start, and only recently Android devices managed to pass all iOS devices.
The 75% to 15% is only for the last quarter of sales, but if the sales can be sustained, and they probably can (they kept growing so far), then eventually the userbases will look like 5:1, too (for phones at least, considering iPhones are only about half of iOS sales).
And Samsung will not fork. They will lose all the apps, and considering they are nowhere near starting their own app store, it's impossible for them to do that within 18 months. I don't know why this idea comes up every now and then. I guess it's the tech press' love for the melodramatic, and from their point of view it would make for great intrigue if Samsung tried that, but when you look at things objectively, that outcome is nowhere near close to a possible reality right now.