In the real world, you have to make choices. A, B, and/or (maybe) C. Each has its set of costs, each has its set of benefits.
Only in la-la land (and Debian) is the answer "don't decide, and just go with all of the above."
"Running optimally" on modern graphics hardware means that you still have to support obsolete graphics hardware, which means your entire decision/design tree has to incorporate that. It's double to triple the initial amount of design/coding work, and exponentially increases the support cost, and does not gain them anything significant on their strategic long-term goals of building a kick-ass *desktop* environment.
They decided to focus their effort into building a single opengl-driven path. For a fallback for completely obsolete hardware, a pure software opengl rasterizer would be used, keeping the complexity at the natural component boundaries, rather than have to have the entire stack aware of everything else. It's sound software engineering practice.
We're at the same situation with desktop 3D as we were in the earlier days of Linux-wireless, before there was solid internal infrastructure and enough common code to make things consistent. Each driver had its quirks, so all applications had to know about those quirks to ensure consistent end-user behaviour.
NetworkManager was a major disruption, because its maintainer took it upon himself to actually fix the underlying buggy drivers so they all behaved in a consistent manner. This is where the Gnome3 folks are at now; a great deal of work is going on behind the scenes to drag the 3D stack kicking and screaming into the modern era.
This means prioritizing development effort -- fix the backend bugs, and everyone benefits in the end, rather than work around the backend bugs, and do three times as much work each time something new comes along.
(and as an aside, modern hardware doesn't even have any sort of 2D engine beyond a dumb framebuffer any more; do we "emulate" the old 2D stuff via the 3D engine, or target the future, and make everything 3D which vastly reduces the overall amount of work necessary?)