Rewriting and retargeting applications is how a lot of us make more money. I am amazed on how many software developers talk about open source and Linux but never talk about how you do some work, package it and make money. Many on here just like to be a laborer and have the man pay you for work and you are basically a slave your entire career barely making real money. You are probably the religious type talking about frameworks, languages, etc. Think about games, game consoles, automobiles, houses, etc. If we were happy with the old stuff and never bought anything new the economy would stagnate. In summary, UA/UWP offers software vendors a new way to sell the same old stuff. Join in and maybe you can buy that Tesla.
Broken window fallacy man. Rewriting the same bullshit over and over doesn't improve the world. This is also why I <3 open source. It's kind of like a baseline "technological commons" to build off of. You should only spend time trying to solve problems that have never been solved before, by anyone. There will always be work left to do as a software dev/computer scientist until we are living in a post-scarce society and exploring the galaxy.