I do think the final years of the previous regime turned a lot of fans off...as did Luck's injury situation and eventual retirement. It's been trending this way for a while though...the secondary market for Colts tickets (mainly the cheaper seats) has been pretty weak for several years...even before Luck's injury. Those 600 level seats are just not in great demand.
Also, intradivisional games (HOU, TEN, JAC) never seem to be as packed as other games. Those are typically the games I go to each year because the tickets are cheaper and I can get really good ones.
I don't know if it's an NFL-wide issue...I went to the CLE/SEA game a couple weeks ago...and it was packed.
i think irsay would want to keep luck and start him and ballard would want to trade him. i get the feeling luck himself doesnt want to be traded either, and about 93% confident hes not coming back anyway