When the iPhone first launched, it wasn't adopted by everyone. I remember when, in ~2009, I and a few friends were the only ones I knew who had an iPhone. It wasn't until around 2011 that pretty much everyone I knew had a smartphone.
The basic business lesson is that you have to "cross the chasm" from early adopters to mainstream users - you can't just start selling a new product to everyone and expect to succeed without massive resources, and even then your product can still struggle (see: Apple Watch). Really, the only way to pull it off is to establish a niche market before targeting the general market. This book  does a great job of explaining the process - Mozilla and Canonical execs would do well to read it!
Sadly, many startups don't seem to have very good business sense and long-term thinking. "If we're not the iPhone yesterday, then we can't compete and we might as well 'pivot' and 'focus' on our 'core offerings'" - not true! There is a market for a user-respecting, fully (or even mostly!) FLOSS smartphone, but my parents certainly aren't going to buy one - they just got their first iPhone last year after all - so sell to me and people like me, not them. Then, maybe in 6-8 years, once the platform has matured and the FLOSS benefits become obvious, you'll start to break into the general market and start to challenge iOS and Android. But I don't think most companies are down for that; sadly, most are just focused on next quarter's profits instead of building something lasting.
iOS = Locked in ecosystem
Windows Phone = Locked in ecosystem
Android = Alternative to iOS
Windows = Alternative to iOS
On a technical level, Windows Phone devices performed around the same as iOS devices (fantastic), but lacked mainstream apps (the same problem as Blackberry) but without the backwards compatibility with Android apps.
Basically, Windows Phone was doomed to be the "not android, and not iOS" smartphone despite the great technical aspects of the OS.
But then came the Windows Phone 10 reboot, with the promise that all 8.1 devices would get an WP 10 update, promise that they eventually broke by allowing only premium devices to get it.
So no, WP wasn't doomed if Microsoft managed to keep the roadmap steady, instead of rebooting left and right.
On the other hand, they seem to be having some luck with hybrid laptops/tablets. Something that most Android devices still do very poorly.
I'm optimistic because I've never seen one of these 3rd option OSes take the approach mentioned in the Crossing the Chasm book linked in my earlier post - they all went straight to mass market vs. targeting a niche, dominating, and then using that beachhead to expand into other markets. If someone makes a legit or mostly-legit FLOSS mobile device, targets it at the appropriate audience, and slowly and patiently nurtures an ecosystem, I think a 3rd option can succeed.
No market is ever truly "won" - they are all susceptible to disruption, no matter what monopolies or duopolies may exist. Not saying said disruption is easy, but it is always possible, and personally I think it is something worth pushing for vs. settling for the status quo, esp. when said status quo is either closed (iOS) or not very user-respecting (stock Android w/Google services).
Now, you are absolutely right that no market is won forever. Like you said, Blackberry was once king of the hill and was toppled by Apple. But again, like you said, Apple had a better product (and the resources to bring it to market).
So now, the question is: what's the next thing? The next "winner"? Where's the better product than iOS and Android today? Sailfish? Tizen? Mer? Fuchsia is in development at Google and looks intriguing, plus it's open source. What else could "win" mobile?
^^ I don't see how the above makes me a Linux fanboy?
As far as the next "thing" or "winner" goes, I'm not sure what that will be. My only hope is that it's not locked down and non-free by default like mobile operating systems have been. However, seeing the way AI and such are shaping up, I'm skeptical that we're not headed for a very closed, proprietary future with few if any viable alternatives to the mainstream offerings of Apple, Google, Amazon, etc.
I don't believe that a "better product than iOS and Android" exists today. Sadly, Ubuntu Touch will never be that better product, nor did Canonical ever really give it the opportunity to be.
So, what could "win" mobile? Given that the smartphone market has basically matured, the only two viable approaches are:
1. Develop a niche product that could eventually grow into a mainstream offering, or
2. Build the next "big thing"
Unfortunately, Canonical tried to make Ubuntu Touch a direct iOS/Android competitor from the get-go and they barely made it to market before everything folded, so I'd argue they tried to take a 3rd approach and failed. Right now, I don't see anyone who's really taking either of the above approaches with much success, but I guess the good news is disruption _is_ coming. When and how, we'll just have to see, but one thing is for sure: Ubuntu Touch is out of the running.