Air carrier pilot reported that although ATC issued alerts for weather balloons in the area at cruise altitudes; the presence of the balloons in vicinity of fast-moving aircraft is hazardous and should be addressed.

Narrative

While in cruise flight at FL340 in the vicinity of Denver heading southwest approaching the front range; DEN Center started issuing alerts to aircraft on the frequency of weather balloons in the area. They reported weather balloons were at altitudes around FL240 and also at FL420 and said if aircraft needed deviations to please advise. They also reported the balloon or perhaps their equipment were 28 inches wide. ATC also mentioned that the balloons were equipped with transponders but that many of the transponders had frozen and failed. I'm not sure how any aircraft flying at Mach .78 would be able to see and avoid balloon or its equipment that was 28 inches wide even it was in clear VMC conditions making these balloons a threat to safety.Obviously an aircraft has already collided with one of these balloons resulting in a shattered windscreen; flight deck damage and crew injury at altitude; a situation that ended safely for passengers; but that could have ended with far worse consequences. How much longer are these balloons going to launched into crowded airspace saturated with passenger jet aircraft? This is a safety hazard that needs to addressed. An obvious recommendation would be to halt the launch of these weather/research balloons into crowed airspace.