• 1. There is no universal definition what a technology needs to achieve in order to be "successful" or "failing". Linux, in particular, depending on perspective, could have either "failed" literally all the time because it hasn't (yet) achieved desktop dominance, or it could have been massively successful on the other hand because it has been dominant on servers and mobile phones (in the form of Android). Now if we look at desktop Linux in particular, it has also somehow "not failed" at the same time, because it has continued to grow. It was stagnant for a very, very long time at around 1% market share but recently it's been steadily increasing up to about 5%. Again, depending on your definition or vibes, you could call this either successful or failing. Which is why these terms in isolation are kind of meaningless.
• 2. Microsoft is a company, Windows Phone was a product by that company. If a product from a company "fails", the company will abandon that product. It's that simple. Sure, gaining foothold against established iOS and Android is super hard. Which is the reason why Microsoft's effort failed. But, they are just a company. Linux, on the other hand, is at its core a world-wide community-developed open source software project (as well as most of the software that runs on top of it) and so it doesn't really matter if it grows up to Android or iOS size. It's still being developed as long as people want to develop for it. There's no single CEO looking at some statistics and calling to cut that project because it doesn't serve his definition of success.
• 3. In general, any project that strives to eventually rival established software products within a market has a steep uphill battle. It's the network effect. Developers develop for iOS and Android because 99% of the user base uses those two mobile OS. Only very few developers will be like "oh there's this new thing currently at 1% market share, sure, let's help it grow!". This alone prevents lots of apps you'd like to see on mainline Linux based mobile OS to ever exist for it. So you need to fall back to some workarounds like Waydroid, to run Android apps on Linux in the meantime, while Linux on mobile continues to grow and continues to attract developer attention. This can take a long time! On top of that are anti-competitive and monopolistic strategies and tactics being used by Google and Apple to ensure they remain on top of the mobile OS food chain. One such example is Google's so-called Play Integrity API, which is basically a form of DRM. Some app developers have been misled by Google's marketing to believe that they should implement it to ensure that their app is running on a "secure" device or environment. What they fail to realize is that Google uses that to basically label every non-Google-sanctioned Android distribution (like Graphene or Calyx or Lineage or many others) or Android runtime environment (like Waydroid) as "insecure" or other negative terms, which then prevents the app from being run at all. Furthermore they plan to restrict "sideloading" which means they want every app to only be distributed via Google's app repository. This means Google wants to exert a ton of control over the developers, the platform and every single app that runs on it. Developers are usually being lured into this via marketing tricks that this would much more secure than it was before or similar nonsense. What they fail to realize is that this also destroys flexibility and freedom for the users to choose what they want to install, and from where. On desktop PCs, you have had these freedoms for forever (even Windows(!) is much more open and neutral than iOS or Android are these days) - you obviously also should have these freedoms for your mobile OS because it's also just a computer with an OS on it. It's simply none of the business of the OS developer to tell the user which apps he should install and from where. OS and apps are completely different things from completely different developers. Choice is being limited significantly when Google centrally controls what apps are being distributed at all, there's 1 company telling you which apps you can and can't use. This is obviously bad and should NEVER happen, but many developers, users and other people confronted with this are easily lured into Google- and Apple-operated cages by fake security talk/marketing. That means they help establish Google's and Apple's monopoly on mobile OS. This, combined with the network effect for app developers, is why it will take lots of time and also not a commercial product (because no commercial product will have the amount of money or time to compete with Apple or Google) to rise up to these monopolies until a third viable option is on people's radars. Linux, due to its open source nature, is the only project that CAN achieve this because it can't fail. It can only grow. But we also need to ensure that at least Android remains a somewhat neutral and open platform. If Google becomes more like Apple controlling literally everything, it gets even harder for alternatives (and for Android users in general).

Linux phones are usable right now, but of course you have some limitations in practice... many apps aren't available or you have to use workarounds. If you mostly use open source applications you could be fine though. Although it's likely that you still need a secondary, small Android-based phone that you turn on just for those rare cases where you absolutely need a certain mobile app and it's only available for Android. At least while Linux mobile OS usage is still low. It's probably going to grow faster in the future, because those monopolistic companies usually enshittify their products and services at some point (Google is already well on it) and then regular Android/iOS users become so annoyed at what they're using that they also open up more for alternatives. It's basically what's happening in the desktop OS space right now - Windows continues to become more user-hostile and annoying to use, and desktop Linux passively (as well as actively) becomes more popular as a result. At some point, these companies forget what made their products popular in the first place and are only operating in the mode of milking users for data and profits, because they don't need to work hard anymore to improve the product - it's already popular enough. At that point, regular users who normally don't care about things like freedoms, privacy and ethics in the product they use will notice that things became worse and might switch simply because of inconveniences they didn't have before.

Another very good option beside Linux-based mobile OS these days is GrapheneOS. It's the best Android-based distribution you can have currently, nothing comes close (not going to elaborate here because long post is already long). But you still should be prepared for increasing hostility from Google towards unofficial Android distributions, and some apps which use the Play Integrity DRM to not work. If you encounter this, make sure to let the app developer(s) know. They need to realize that they are only serving Google's interests with this, not their own.