GhislainG :
I really doubt existing games will be rewritten for DX12; it would be much easier and certainly less costly to patch them for DX11.3 compatibility. Because of its complexity, you can't expect all new games to be DX12 compatible (just like most current games are not Mantle compatible). Releasing new DX12 versions of a game makes a lot of sense because those that are CPU bound will benefit from it and likely be willing to upgrade.
I don't expect anything from devs apart from greater size, cost & component demand of new games, GhislainG.
I understand it will be up to game devs as to whether their games are DX 12 or Mantle compatible. What I've very little idea of is just how complex it is to make any game compatible with either. I imagine it could be a relatively simple piece of coding, as it involves a sort of language translation at a fundamental level, but that's just the imaginings of a coding ignoramus.
PC gaming is probably overtaking Console gaming at present, for many various reasons, but is a pain for devs because of the complexity of differing system builds, which, of course, both DX 12 & Mantle, attempt to address. When you consider the added growth in Mobiles gaming, & how both DX 12 & Mantle address the power consumption factor too, then I think we have good reason to at least hope for, if not expect, newer games being compatible.