You are avoiding the issue that the good Member Dale has raised.
Why 6 runners?
Why not 7 runners as well?
And 5 Runners?
Both 5 and 7 are small Fields , maybe not so much 7.
But why not 5?

In your long reply you argue that "How many times has one seen the rank outsider get up in 5 horse races?"
Perhaps this might be worth testing in UB's System Builder.

2 Rules
Rankest animal
5 horse Field