I don't think the ideal game engine would run on all crappy platforms in ideal mode. I would be easier to say that the ideal game engine runs only on the ideal platform in ideal mode. That's how consoles work too.
On the other hand, it would be also nice that the same code runs on all crappy platforms, so the ideal engine should automatically remove stuff which don't work on lower platforms.
On the other hand, it would be also nice that the same code runs on all crappy platforms, so the ideal engine should automatically remove stuff which don't work on lower platforms.