That is true but Delta still sells Main Basic fares. If it didn't care or want to sell them, they wouldn't. The government doesn't require that they sell it. That means that Main Basic fares are of some importance to Delta. Certainly it's not as much as the big spender.
Main Basic fares are no longer bargain basement. They are the old regular economy fares. Furthermore, Delta seems to encourage the non-Comfort, non-First class passenger to buy Main Basic because a higher fare offers very little besides some SkyPesos, seat assignment in advance, and slightly earlier boarding.
The only importance these fares have for DL is: 1) FIll seats that would otherwise go unsold, and 2) Act as a hook to get referrals from Google Flights for passengers to then say "nah... I want main" and then pay the additional fee. Re the "very little" that Main offers over Main... I won't fly without the seat assignment and guarantee I can travel with my cabin luggage. I won't fly middle seat for a flight that's more than an hour.
But your argument is missing the point: Delta doesn't really care if the passenger who buys Main Basic selects them over Spirit. They know that that passenger is flying the cheapest fare they can find. They'd rather take the passenger who will pay a little more... and they also really only want to "reciprocate" loyalty to someone who does this consistently. From Delta's perspective, the passenger that won't even pay for a seat assignment
is a bargain basement flyer. It doesn't matter what you think about the fares.