if we didn't have the spam we all hate, most blocks would be mined empty and miners would earn even less than they do today
You make it sound as if miners don't earn a lot. They earn over 50 million dollars per day now, and a large share of those Bitcoins must be sold for dollars to pay for hardware and electricity. If they earn more, all they'll do is buy more hardware to increase their share and increase the hash rate, but more money leaves the Bitcoin ecosystem.