This is exactly it, Spider-man is Sony's cash cow so they keeping on announcing new movies so they can hold on to the rights. In the long run it was a stupid decision to do by Marvel but during the time they signed that deal they were almost bankrupt so it was good for them to get some quick cash at the time.
Marvel did what they had to do at the time. Although I'd much rather see Spider man on his own than as a member of the avengers which is what would happen if he was with Marvel/Disney. And I don't know if Disney would allow their director to kill Gwen Stacy