Streetcars would have likely gone away in many American cities without GM, but GM and organized crime certainly hastened the demise of streetcars. Public transit was just becoming unprofitable due to automobile subsidies and turning the private transit companies into government owned industries was inevitable.
https://www.startribune.com/streetcars-buses-minneapolis-st-paul-gangsters-kid-cann-fred-ossanna/600097062/Here's an article about the demise of streetcars in the Twin Cities that states organized crime led to the demise of streetcars.
"The short answer is yes, the Twin Cities underworld was involved. But the conversion from streetcars to buses was likely going to happen regardless, since the rise of personal automobiles had gutted the profitability of the region's transit system.
"It happened years sooner than it otherwise would have. And crimes were committed," said rail historian Aaron Isaacs. "If this had been done the normal way, crimes would not have been committed."
"Isaacs, historian for the Minnesota Streetcar Museum, is not conspiratorial about why the streetcars disappeared. The majority of them were antiques, he noted, and many people wanted them gone because cars couldn't pass them on city streets.
"People have asked what would have happened if the gangsters hadn't come in, and all of that," Isaacs said. "And the answer is by 1960, I think the streetcars would have been gone. It would have happened, but it would have happened slower.""
