Sounds easy on paper, but I think Tizen would be a disaster. Samsung customers are like Microsoft customers. They didn't care about performance to begin with, which is why they chose Samsung (or Microsoft) in the first place. Once they learned how to use their shitty software, the worst thing you can do to them is make them learn new shitty software. Look at Microsoft XP customers. MS can't get them to upgrade over a decade later. Every new MS OS release is followed by years of bitching. While quality shares part of the blame, a big part of it is the customers. Think about it, if the Samdung customers were smart enough or wealthy enough to select good software, they wouldn't be using Samdung or MS in the first place. The galaxy S owners over the age of 50 are the one exception. They need the bigger screen to avoid using reading glasses.
Also, Samsung just tried Tizen on some of its watches and it didn't go well. They are now going to release watches for Android. If Samsung experiments with Tizen on the Galaxy S phones, I think it will be very short lived.
I am going to go ahead and guess you have little experience actually using android, let alone a galaxy s5. there are some really nice things one can do with android and on the samsung s5. And let's face it- neither of us know much about how great Tizen can be, as i doubt you touched the watch that had the software. A watch is not a good identifier of how the software would be on a phone. The best thing Samsung could do it to distance itself from android.
How great was iOS on the iPod shuffle? How well did that sell? Let's not pretend that Apple has never had a flop. Great company? No doubts, innovative and pushing hard for the future, i love every Apple product i have ever bought. But you cannot presume to know about Tizen and Samsung's choices regarding the software because no one bought the watch it was on- nearly no one bought the iOS laden iPod shuffle with a screen, and so Apple changed it back.
There are probably more power users on android than there is iOS, because android lets you roll up your sleeves and do some extra.
And microsoft is not even in the same league when it comes to the smartphone sphere, they should not have been brought up in the first place. The performance on the S5 is better than any other android crap i have ever messed around with.