Had Delta not participated in the A320 life extension program, I understand many A320s would have reached their hours limits by now. The extension indeed doubles the hours limit, less so the cycles limit. Thus, a few of the oldest planes will apparently still "cycle out" sometime in the next few years although not without earning good revenue for Delta to the very end before finally heading for part out.

As you touched on, seven Delta A320s were retired to the scrappers in 2017 but none since then. Although old, I get the impression these seven planes still had some life left (someone correct me if I'm wrong) and the decision to phase them out seemed almost last minute. None of the seven got the upgraded interiors. Just a guess, but perhaps these seven would have run out of cycles before new interiors (with additional seats) would have earned back a good return on investment. Update: I stand corrected, as posts on this thread about two years ago mention that some A320s got P1 cabins but were retired without receiving P2 mods. So actually none of the seven retired A320s got P2 cabins. Compared to the earlier Northwest Airlines layout, P1 birds had ten additional seats shoehorned in (as opposed to seven on P2) so perhaps the retired P1 planes still had good opportunities to earn a decent Return On Investment before phase out.