UWP is a platform with a future, regardless of smartphone marketshare.
I don't see updates to windows 10 mobile stopping yet either (for a start there are devices that are not EoL, but see below). We have two very clear announcements from MS, that do no depend on mobile profits -
+Cshell - MS wants its UI to scale automatically across devices. So win10m style on small devices, tablet style, console style. Every single OS with onecore (console, PC, tablet, IoT core, and mobile) will get this update, so its not remotely connected to the success of windows 10 mobile (And it sort of depends on it, seeing as they wish to merge all the UIs)
+Cortana skills api - The Cortana skills api is due soon, this brings in microsofts easier to program and popular Microsoft bot framework, as well as the ability to repurpose alexa skills code. Cortana is pretty cloud based, but Cortana runs on everything at the top, right down to IoT core (which as yet, makes MS no money).
It's very likely windows 10 mobile will be retired one day. But its useful as a platform for developing cshell, Cortana on mobile devices. MS wants to get into everything including smarthomes and smartwatches, so there is every reason to keep working on these form factors in some form, regardless of current profitability, as future investment.
I don't see them discontinuing updates on mobile this year at least. And if they do, it would only be to port those same features to WoA - but seeing they haven't prioritized mobile, and won't likely be releasing any phones with it any time soon (if at all), win10m seems like the better beta/testbed for those desired changes.
And as I said above, regarding UWP apps, people use them on tablets and notebooks, on xbox and on desktop. The windows store is actually growing despite what you'll hear from mobile users. At worst for mobile this may mean a slight dip in mobile centric apps, like gps, banking or messaging focused apps. But with windows on arm coming out, a platform that is also LTE and calling enabled, also includes GPS, the existence of tablets and notebooks with these capabilities will probably breath life back into those too.
For example, WhatsApp will probably want an app updated for LTE enabled notebooks. With 500 million users, and notebooks being the fastest growing catergory of PC, something 1 in 4 or 1 in 5 people own - there is commercial value in being in that space.