That would be lovely except for the fact that all middleware currently available is sold 100% on the premise of "rendering engine" being only a tiny fraction of a giant monolithic package, not to mention majority of productions rewrite the rendering part for one of them almost as a rule.
The greatest myth that Software Industry created is the one about monolithic do-it-all framework, and in games the downsides of this only got magnified.
Not to mention that scope for what constitutes a "game-in-box" keeps growing, and at an increasing rate.
Don't really deny that, but at some point given the complexity of the making the whole package, ie the renderer, tools, art pipeline, etc, it may just become cheaper and/or more beneficial for people to ditch creating/supporting all that in house and just go with middleware. Why would a publisher bother supporting multiple technologies across multiple teams/ips/games when a package like Crytek offers can do it all, and be cutting edge with realtime gi as a bonus. That would even unify all the teams on the same toolset which would make shifting talent across projects far easier if the renderer/tools/art pipeline was all unified. In the old days that meant compromise, but with Crytek and Frosbite being at the cutting edge of basically all tech out there it's no longer the case.