It will also be interesting as Basic Economy fares are rolled out more...there could easily be a case where someone on a Basic Economy fare has a seat request card at the gate, and only C+ seats are available -- will GAs just give them C+, or since BE is supposed to mean no C+, will GAs move someone else from Y to C+ to make room for the BE passenger?

I suspect they won't bother to make the move, not least because the passenger moved up to C+ might not have wanted it for all of the reasons discussed upthread, but the result will be that on elite-light, leisure-heavy flights BE passengers will probably pretty consistently get to sit in C+ (even more so if speculation in other threads of Delta adding more C+ seats to some aircraft types comes true).