When and if DL announces it, they may use the same "reason" as AA did: that "it wasn't popular enough" and that "too few customers booked RTW awards." (BTW, AA announced it on the morning of the night after the RTW OW award was removed.)

That does have a ring of truth, as most are probably totally unaware of it. For example, I was and only found out about AA's OW Explorer when I was trying to book three one-way flights and the agent told me it'll cost less as OW Explorer vs. as three one-ways on one PNR, as it was all going in the same direction around the globe.

However, that is also precisely the reason not to remove it. The best awards are aspirational awards such as RTW that few passengers ever book, yet more aspire to and save up miles towards. The downside (actual use of it) is likely far lesser (few and far inbetween pax actually book RTW) than the upside of having the aspirational award that encourages more loyalty as pax work hard to rack up enough miles for an RTW award, that to many may remain just that - an unrealized aspiration.