I'm not sure one could over work bees. I could be wrong, but I just don't think they can be forced to do more than they want or can do.
I guess a bee keeper could close off access to the hive until the job was done, but I don't think it works that way. From what I seem to recall, bees come and go, back and forth between the hive and the flora.
It would be pretty cool if you could train a bee. Not only would the roll over, shake a paw, sit pretty, etc. be extremely cute, they would make for a wonderful property protection service.
On a more serious note, there certainly are more pollinators than just bees, that's true. But if large colonies of bees disappear, I imagine the population of other pollinating insects would grow, upsetting a possibly delicate balance and perhaps increasing the amount of infestations that occur from time to time, often with devastating effects on formerly dominant, native species.
Oh, another thought just entered my mind. I wonder what affect global human consciousness has on bees. Is it possible that a rise in human consciousness and awareness can impact bee populations in a positive manner? Sorry for the stream of consciousness, but I had never really considered the effects of human consciousness on biology, both in general as well as on a species-specific basis.
Gonzo