It'll start as "better on DirectX 12," there'll be a Crysisesque "DirectX 12 only" game that is the prettiest thing anyone has ever seen, everyone will move and the holdouts can keep playing old games off Steam and GOG if they don't like it.

That was the plan with DX10 and we have seen, how that one went.

The thing is, the majority of gamers will update based on the available cash, and so gamedevs will be targeting the expected hw base at the time of release.

So it really depends, whether you except the gamers to go on spending spree (nvidia and intel would certainly love that), or not.

Existing DX11 cards will be compatible with DX12.

Except gamers will upgrade because of the performance implications of the DirectX 12 API. With an incentive like that, no one will care about the few holdouts that refuse to upgrade because they're uncomfortable with a few UI changes that don't affect what you can actually get done.

You mean like when Microsoft tried to force adoption of Windows Vista by not bringing DirectX 10 to XP, and in response developers stuck with DirectX 9 for years?

A huge portion of the gaming market (China and large parts of Asia) is still using Windows XP. D3D9 renderers aren't going anywhere.

No, DX9 renderers aren't going anywhere, but they're being abstracted away and all the shiny new code is being written for DX11 and DX12. So all of the fancy high end features will be on newer systems only, with the others having less and less support. I know because this is what I'm doing at work at this time.