When they were specifically the cause to the lack of those targets, due to constantly attacking the same hostile to the point where that player felt that his game wasn't getting any where. So they quit the game. This I have seen happening in a lot of online games especially when it is a militaristic game. If they were to take a more liberal approach by having several hostile targets and juggling them to allow those others to grow and feel that their game is progressing. They are more likely to stay around. If a farmer over milked his cows they would go dry on milk too. The same goes to exhausting your targets to the brink as well.
As for the resource protection of the researcher :- Maybe we could make a function similar to the Sirius Defense Up and down function. Where the protected resource's have to be lowered into a Tunnel System and brought up from the tunnel system to actually be used. This would allow for a period of time where the resource's could be attacked as they would be considered un protected while in the up stage but once completely lowered the resource's are safe.
This would lead to the researcher having to be actively be involved in the resource protection part to their spec. By downing the resource's to protect them and upping the resource's to actively use the resource's or to load onto ship's. This would then give a hostile small windows of opportunity to raid resource's.