There's plenty of 600-40/50x+ being shot though. It's pretty rare that a mid-range match (here locally anyhow) doesn't feature multiple cleans. Heck, the 300 yard aggregate winner last year was 165 shots clean with 130 X's. A single point dropped (or even too many 10's!) and you might as well go home. The 600 yard championship (80 shots) featured the top three clean (800), with one or two X's of separation. Our monthly 500 yard matches are generally only won if you are clean (600) with a 45+ x-count. I personally find it boring which is why I use mid-range matches mostly for testing.
But, I get that a lot of guys enjoy it, everybody has different goals. I would prefer a smaller target for the challenge, but don't really care one way or another as long as there are good long range matches available to keep things interesting.
EDIT: The idea of using e-targets to shrink the target is actually pretty interesting. I wonder how hard it would be to get SMT to offer reduced-size targets for non-approved shooting.