While we're on the topic of non-mainstream OSes, there is a reason why Firefox OS, Ubuntu Touch and even the once promising Windows Phone have failed: the market brought up by Google and Apple together is simply too big for potential competitors to handle. It's not that alternative OSes are worse (I myself am quite fond of Sailfish OS) but Google and Apple's de facto duopoly means competition is heavily stifled.
None sense, when Apple launched the iphone their market cap was $279B fixed for inflation. Its not that others do not have money to compete, they are just too afraid to compete. They go for greener pastures where a standard has not been established which is a caveat in the capitalist system.
The reality is that it's just not enough to ship a smartphone OS these days and call it a day. You need to own and control your own ecosystem, and the costs and efforts requires is simply too prohibitive.
again none sense, such money exists. They just decide not to compete and use their money where they feel they are likely to make more back in investment.
Second is that developers simply don't want to support another platform.
this is the main one. I do not see say Adobe building 5 version of Photoshop to support 5 or more competing platforms, but if we can assume that android will be the standard then multiple OS variants can support running the "Android" app.
Third, what even would the selling point of a third smartphone OS be? How would this differentiate itself from iOS or Android?
This is not an Apple or Google problem. It's one of simple pragmatism and economics.
doesn't have to differentiate too much, its an option to choose which company or product to support. I would assume for example that if EU members had an EU made OS they would opt for it over the American one. Another thing is privacy, both android and Apple collect data. Another thing is consumers not creating their own monster, we do not want to live in a world where 1 company controls a whole market segment.
If we went with your logic, we can say what can a toyota offer that a volkswagon does not? what would Ford offer that hyundai does not? they are all cars that take you on the road. Same for light bulbs, tires, tv screens, ssd storage, and almost everything else.
The place a competitive third OS would likely come from is China. But American regulators are working overtime to make sure Apple doesn't have to compete with China.
Apple loves the regulations that let them sell in a China-free hug box. Government intervention for me, but not for thee.
China already have working HarmonyOS which according to latest updates is all native and no longer contains Android code.