That's allegedly what happened with QB, and it shouldn't happen with games like QB. But there are countless of applications and a application concepts where this approach is perfectly viable.
There's nothing about UWP that requires developers to ignore the differences between the platforms they are targeting. Some applications, like graphics intensive games, require more work to target specific platforms than others. This fact doesn't mean that the concept as a whole is undesireable.
If you were making a Space Marine 12 stat tracking app, you'd like the idea of coding it once, and easily porting it between Multiple hardware configurations.
The market wouldn't particularly care whether you used UWP or not. But the developer would benefit.
Now, If you are making Space Marine 12, you could still use UWP (maybe because you want to use XBL for networking, or communicate directly to a UWap stat tracking app) but should also give the Xbox one version and the PC version more special attention. The idea that the code once concept is inherently flawed is just wrong. It just isn't suitable for every single possible application. Outside of the relm of heavy video games, and design software the vast majority of application are prime for this type of approach.