We KNOW that DX12 works better with AMD, but we might end up in a vicious cycle in which no gamers buy AMD because games run better in DX11, and developers develop for DX11 for PC because the gamers owns mainly Nvidia, GPUs, especially since DX12 is more difficult to implement (parallel programming, more control etc)

It has nothing to do with Nvidia and everything to do with the fact that DX12 is a pain in the ass to work with.

Lets with what IO Interactive said on the matter, the devs of an AMD SPONSORED game (Hitman):

The interface of the game is still based on DirectX 11. Programmers still prefer it, as it’s significantly easier to implement.

Asynchronous compute on the GPU was used for screen space anti aliasing, screen space ambient occlusion and the calculations for the light tiles.

Asynchronous compute granted a gain of 5-10% in performance on AMD cards, and unfortunately no gain on Nvidia cards, but the studio is working with the manufacturer to fix that. They’ll keep on trying.

The downside of using asynchronous compute is that it’s “super-hard to tune,” and putting too much workload on it can cause a loss in performance.

The developers were surprised by how much they needed to be careful about the memory budget on DirectX 12

Priorities can’t be set for resources in DirectX 12 (meaning that developers can’t decide what should always remain in GPU memory and never be pushed to system memory if there’s more data than what the GPU memory can hold) besides what is determined by the driver. That is normally enough, but not always. Hopefully that will change in the future.

Read the bolded letters in particular.

Timespy shows that DX12 itself will work just as well on Nvidia as on AMD.