I think my biggest question here is... why?
It does seem as though the code changes to make the game better are out there, but Mojang isn't implementing them. Like you mention in one of your linked posts, simple things like fonts can be optimized just by changing a few lines. Is there any reason that Mojang intentionally keeps slow code, or has Mojang just laid off all its staff that know how to write OpenGL rendering code in favor of more people who can design new blocks? Or are they just intentionally lazy?