If however you are none of these, then why should it matter to you that Windows mobile sales have fallen?
well. All the three mobile OS now are app centric. Apps are the things that do matter. If I am "user", I would like to use it as a daily driver, wish it to help me do all the work I can do on a smart phone the other two mobile OS' letting others to do. If there are very less number of users around in a platform, If I am an app developer, I may not develop for it. So for any user who is using Windows Phone or mobile, these numbers indeed matter.
You can say that universal apps will save mobile by pulling all people to develop apps there. Even if all the apps are developed as Universal apps(it's not), I don't think the developer will go the extra mile to support the mobile app that is used by less number of people. So, if it is eligible for mobile as app(it should if it's universal), the experience will be less than perfect. So for any one to want better experience using the phones, these numbers indeed matter.
If you are the odd man out in a group to have a Windows phone, it's fine. But to have a phone that has only a fraction of apps or with apps that are way better on rival platforms, it's not fine. You will be a laughing stock if you buy such a phone selling at a higher cost. So as a user, it does indeed matter whether it is as powerful as that of others.
Oh, by the way. I am not bothered about Windows Phone dying. I use very little apps and am convinced that Microsoft will continue developing Windows 10 mobile. Even if Microsoft pulled the plug for Windows 10 mobile, I will not be worried. I will keep using my phone until it is useful and will just move over to any other platform that I find good for me.