If bees were prevalent before, their mating habits would have no effect on declining population. What is probably happening is they are losing natural habitat due to overuse of chemicals so this is killing off the populations slowly. It is not as if their mating habits have changed in the past decades.
Male bees have always been unable to mate more than once, while bee populations have been stable or growing until recently. The decline in bee populations is due to the wide use of Neonicotinoid Pesticides. According to a study published in Proceedings of the Royal Society B, these pesticides reduce male bee sperm counts, hamper their homing ability and produce enlarged ovaries in female bees.
The fact that a male bee can only mate once is not a contributing factor the the declining bee population. That fact has not produced a decline in the bee population for centuries. We, as humans, need to realize that the chemicals we use are most likely the cause of their decline and address that.
While it is true that a male bee can only mate once, this is not a major cause in the declining global bee population. Factors such as climate change and pesticides used in industrial agriculture are the major reasons why bee populations are dropping worldwide. Exposure to insecticides has adverse physiological and behavioral effects on bees, who are not even the intended targets.