Samsung isn't going to ditch Android any time soon. Tizen will be offered in low cost mid range phones at first to see if it takes off. If it does, then I'd worry but I don't see it happening. Who knows, it might be only offered to emerging markets. But to note though, if Samsung DOES ditch android, It will either be Moto, HTC, or Nexus for me.
As far as Android becoming more of a locked down OS....I don't see it happening. In fact, Google has been releasing it's apps to the play store giving users more choice instead of ramming it down your throat. They are however, trying to get more of a unified look and unified experience. This won't hamper app development but yet make developers work a little harder than writing sloppy code just to make a few quick bucks. I think they are "trying" for an apple approach to..."it just works" and I'm all for that.
The only thing that is irking me though is the fact of ads. They slapped adds in Gmail and a few other apps over the years making it cheap feeling. I know Google is an advertising company but slamming ads in every nook and cranny they can think of is a no-go for me. Just look at the new dialer in 4.4. Google says that's old code. OK. Then why hasn't this been mentioned before in other iterations of the dialer? I think they were/are planning to add ads to the dialer in some fashion. My god, Sprint added ads to the voicemail app....ridiculous.... and it's 1.99 a month to get rid of them!
I love android. I do. But more and more ads are making it look and feel cheap. I know I can always root and rom but I shouldn't have too. With all this said, Samsung needs Google as much as Google needs Samsung. Period.