While Tizen probably is theoretically a viable option for an OS, it's been official for 7 years. And only 5 phones have run it on Samsung devices. 5. If Samsung truly had the power that they did, they would have crippled Google about 5 years ago. I think they already knew then that launching an OS isn't as easy as it sounds. I recall hearing that Samsung was offering incentives for people to develop for the OS, and there weren't many takers.
To quote a great Jedi Master: 'Size matters not.'
Google and Samsung are in a symbiotic relationship. If Samsung were to walk away and try to go all in with Tizen, it would hurt both, and severely. Samsung probably a lot more than Google. Once people found out the app selection was minimal, they probably would be returning devices so fast Samsung's head would spin.
Not really a fair comparison. Launching an app on iOS is a feasible option, as there is a solid user base for the OS itself. In Tizen's case, you would need to convince users to leave what is familiar to try something new and different. There is a user base who would do that; they are prolific on forums, like us. And even then, not everyone would be willing to do it.
In general, the only company that has the ability to try and take on both Apple and Google for an OS would be Microsoft. They have the resources, both technical and capital to do it. If they could do it by creating their own OS and still being able to use Google Play Services, they might have a chance. However, they tried and failed....miserably.
I am not saying that neither one will topple; it would just take a lot of things happening at the right time for it to all work out.