Why can't we have a new mobile OS?

So, do any of you remember pre-iPhone that we had a lot of companies in the mobile phone market (notably, Sony Ericsson if I remember the name right, Motorola, BlackBerry just to name a few) with their own OS or something?

Why did, post-iPhone, we get locked down to Google's ever so present Android and Apple's rather limited iOS? I mean, we had Windows Phone (which eventually got upgraded to 10 Mobile) from Microsoft who had features like Wi-Fi Sense (or a function that shares your Wi-Fi password with your contacts - that saved you a lot of trouble) and Live Photos (Apple stole that from them).

But sadly, Microsoft entered the OS game too late and as such, Windows Phone didn't last long (Nokia made some beautiful phones, though), so, now it's in 'maintenance mode' (to put it simply). That, and the issues that it has caused the user base to move away to another platform.

It sucks that we're locked in a duopoly between Google and Apple. And here's the thing:

Google will either try and not offer their services in an attempt to kill the new OS or offer their services because they know they'll get valuable data out of that OS' userbase. Apple... not so sure about them.

But there's some good news, at least, if you're conscious about privacy:
Purism is currently working on their Librem 5 phone. Which means that, if they get this phone out successfully, we might have one more option to consider.

I'd like to also mention that LineageOS also exists. While it's technically still Android, it doesn't come with Google services and is more respectful of your privacy, although, the amount of phones they support is a tad limited but not that bad. Best part: It's free and open source.

If you can't live without Google, you can try LineageOS with MicroG. MicroG is Google Play Services but reimplemented to be open source and if you're curious what F-Droid is, it's a store for free and open source software. LineageOS with MicroG is also free and open source.

There's also the /e/ foundation. As far as I can tell, they're using their fork of LineageOS in order to make a Google-free OS. They also have their own services. In addition, because their fork is bassed on LineageOS, they have some devices that LineageOS supports! :-)

What do you think, should we have more mobile operating systems to choose from or should we encourage manufacturers to use LineageOS?