and yet US, UA, and WN spend more of their total maintenance dollars on outsourced maintenance than does DL.
those are facts that are backed up by DOT data which is provided by each airline.
US obviously is completely unrepentant as it sends its outsourcing figures to the DOT every year.
The reason why AA and US don't spray their planes is because they don't fly to, wait, wait, Australia which requires that the planes be treated someplace - and the US does not allow it to be done on US soil. Having been on foreign airlines on which the FAs spray the cabin with passengers on board, I can absolutely assure that the vast majority of DL passengers do not want to be fumigated in their seats.
if you come up with a better plan for DL to spray its planes, be sure and let DL know... they might cut you in on the money they have saved.
If you think that AA isn't flying empty planes to/from China for maintenance, then you are naïve and AA is doing it whether you believe it or not just as does every airline that does heavy maintenance overseas.