Thanks for the numbers, they definitely shed light! You’re assuming the ~100% cartel is too polite to reorg out for dominance, of course.
Given the widespread awareness of a 51% attack, I think it’s uncontroversial to write software optimized for miners who control less than that.
(Complete disclosure: I’m with Peter Todd (IIRC) that small miners are more valuable than large miners, so I’m biased to optimize the system for them with my volunteer efforts!).
But even without this bias, 50% discount degrades so fast, it’s not far from 100% really, which is far easier to implement. After 7 blocks the error for a 50% miner is < 1%. A 50% miner has resources to change this if they wish, but the expected return would be minimal.