In fairness to the original poster, on most routes the differential cost between Basic Economy and Main Cabin doesn't make up for the cost difference between Main Cabin and Basic Economy. Obviously it varies depending on exactly where you are on the fare buckets, but in general Main Cabin buy-up out of a hub is typically around 20-30% of the ticket cost. Even for a Diamond which gets about 13% "back" on a ticket (11 SM per dollar at 1.3 CPM after deducting taxes from the MQD base) it doesn't equalize the price.

Of course, it does blunt the "upgrade" cost and being able to select a seat and the other soft benefits (IRROPS priority, call center priority, SkyPriority check-in and boarding) has its benefits - although those occur independent of Basic Economy ticket status, although maintaining status is made tougher if you're booking BE tickets all the time.

At the end of the day, Delta is just trying to do what is in their financial interest. Some Choice Benefits directly cost Delta money (Amex Credit, travel voucher, and SkyMiles... in that order). Others - like gifting Gold Medallion status - cost Delta basically nothing (other than SkyMiles on flights a person would have taken otherwise and maybe the cost of the mailer if they still do that for lowly GMs).

And, arguably, selections like GUCs and GM gifting can potentially have some revenue upside (for GUCs - Diamonds booking a W seat that would have gone unsold to get upgraded into a J seat that would have gone unsold versus just buying a Y seat, or alternatively not even taking a trip at all... for GM - getting people more embedded into the Delta ecosystem and taking more Delta flights).

So if Delta can make the "free" ones better and have people choose gifting GM - essentially free for Delta - instead of selecting a travel voucher or SkyMiles or Amex statement credit, that "saves" Delta money. It's a no-brainer / win-win for them from a business perspective.