Sure. I get what you're saying. To be absolutely fair, I've never used TouchWiz as a daily driver, so I may end up agreeing with you.
Just a few things:
-I think split screen isn't meant to be a long-term use feature. You're not meant to keep split screen opened for a pro-long period of time to use as two 2.5" screens as you describe. I imagine it being meant to just quickly and conveniently share, look at, and/or apply information between two apps. The classic example being an address from Chrome browser, and imputing it into Maps on the split screen. Can you just copy and app-switch and paste? Sure, but this is a little bit faster and a little bit more convenient because both apps are opened at the same time allowing quicker access to each app. I know I've found many times where I wish I could do that.
-Smart stay is optional, no? You can turn it off if you don't need it? I've found in my experience more than a handful of times the screen going dim as if it's about to turn off. My main concern with Smart Stay is exactly how good it will be at acknowledging my eyes that I'm staring at the screen. Also, is Smart Stay the feature that knows when you're in portrait or landscape mode, and will keep it that way based on your eyes? I thought i read something like that before but I'm not sure if this is the same feature. Because if it is, that too, I've found could be useful. Like if my device is tilted a little, but I still want to read in portrait mode.
-I'm okay with the colors of TW. I think they play well with Material Design. But I completely understand if someone finds this too cartoony. The Theme option will be nice for those who want to change it up. iOS doesn't offer this; and how many people have cried iOS' recent design overhaul is not their cup of tea?
-As for the features in relation to bloat, the verdict for me is still out on that. I own an iPhone, and every once in a rare while, it'll hiccup and lag, too, just like any other smartphone I've ever used. If TW minimizes this occurrence as much as reviews have said, then I'll be happy. But you're right, it sounds like TW still has room to improve on that front and I'm glad Samsung is finally embarking in that direction. At the same time, I'm glad Samsung (and others) are pushing extra features and actually influence stock Android. I find stock Android to be a little bit boring.