Originally Posted by
Bowgie
The fact that I wouldn't fly Spirit at a zero fare is entirely beside the point

It's actually not besides the point at all. Delta feels they have a premium product, especially as compared to Spirit, and you seem to agree. Therefore you should be willing to pay more for that product.
Besides, why would Delta want to drop their prices to compete with Spirit on SAN-LAS via LAX when they can profitably sell SAN-LAX and LAX-LAS?