Not sure that is necessarily going to be the case, since we aren't seeing evidence that Comfort Basic is going to be cheaper than what regular Comfort was yesterday. If anything, the additional uncharge to avoid a middle seat might make buying up to Comfort less appealing.
Well, I just did a test booking on the route mentioned by a poster above - and in this case it was clear that it was actually cheaper.
In fact, the pricing positioning was.. interesting. DCA-MIA round trip December 9 to December 16th (by order of price):
* Main Basic - $137
*
Comfort Basic - $197
* Main Classic - $207
* Comfort Classic - $267
* Main Extra - $375
* Comfort Extra - $447
* First Classic - $1253
* First Extra - $1613
Comfort Basic was actually priced less than Main Classic. It wouldn't surprise me if Delta is actually trying to convince people to buy up from Basic Economy rather than buy up from Main Classic. I'm sure it will be route and supply/demand specific, but Delta probably views the middle C+ seats as "slack revenue" right now - outside of business traveler edge cases (packed plane, only middle seats available anyways).
The mileage earning and ineligible upgrades is interesting - psychologically I think they are trying to get elites to keep buying Main Extra (although giving MQDs eats into that slightly). If they can get even a few Basic Economy passengers to upgrade to Basic Comfort, they are coming out on top. In my experience, on full flights with a lot of Basic Economy passengers, a lot of Basic Economy passengers get upgraded into middle C+ seats anyways.
At a minimum, this is consumer psychology crap to try to extract additional revenue. They get to advertise a reasonable looking C+ price to get people to look at the offering, and then anchor people to try to buy the middle option because it looks "reasonable" in terms of what you get versus the opposite ends (Basic and Extra). Even if it only gets average revenue up 1 or 2%, that goes straight to the bottom line.
Personally as a consumer the complexity is getting too much for me. Not seeing the actual prices (regular Main Cabin / Comfort+) on the buying screen annoys me to no end. At this point with what will end up being 9 total sales options (Basic/Classic/Extra for each tier, even if the Extra doesn't really ever count as one - it is again just used as price anchoring) is ridiculous. At that point you're basically just in ULCC carrier pricing models except with a menu of 9 bundle options rather than 7-8 al a carte add-ons. Absolutely ridiculous.