They don't have to fund that. The idea is to make use of the work of the open-source community. The main contribution, software-wise will be pushing forward Matrix, pushing forward open drivers, and designing a platform that makes it easier for others to build apps.

Plasma Mobile is one example of an existing open-source project that aligns with their goals:

https://plasma-mobile.org/

Ubuntu Touch (now maintained by UBports) is another good option:

https://ubports.com/

Windows Phone had a good use interface, many people (including me) prefer it to the horror that is Android UI. Lack of an app ecosystem was probably Windows Phone's biggest problem. I think a possible solution to that for Purism (I was an early backer of this campaign) is to focus on web apps. I know I'm not alone in being thoroughly sick of everything about apps and app stores.

If they have even partial support for QT and GTK they can just use existing desktop apps. Clunky, but it helps.

>Windows Phone had a good use interface, many people (including me) prefer it to the horror that is Android UI

You were definitely in the minority. The one common link between Windows 8, 8.1 and Windows Phone was that hideous and universally rejected tile interface abomination. It was so vile and user hostile that Microsoft had to eventually hide it behind a button in Windows 10.

The tiles worked just fine on an actual touch device - it's not really any different than what any of the Android mobile devices I have features.

It was hot garbage for a desktop/laptop OS, and even more insane for the server OS.