Asteroid research drones disappearing

[JasonMK]

This didn't seem to happen initially, but at some point, my research drones started disappearing as fast as I can create them. I'll be watching them on the asteroid and they just disappear. Is there a max number you can have or something like that so that they disappear when a new one is created?

[Lothsahn]

All worker bots in final factory wear out over use (combat bots and inserters/haulers do not). Your base needs to continue to produce them. Note: it's based on use, not time, so they won't randomly disappear--they have to do be doing something.

This is good feedback -- we need to make sure this is self-explanatory.