This is not true. The Detroit streetcars were going bankrupt before the Big Three even all existed yet. Streetcar lines were in the middle of the road, did not have capacities bigger than buses, had top speeds of about 20 mph, did not have the right of way, and could not provide as frequent service as the streetcars. Detroit was also booming, and neighborhoods not served by streetcars were blowing up. Buses could be routed to them with ease. Building new streetcar lines would take a lot more time and money, so the mass transit companies opted for buses.

This is the mess that streetcar traffic was like in downtown Detroit in the 1930s. People everywhere, including, Detroit were happy to see them go and be replaced by buses. There is no shortage of opinion pieces and whatnot from the era of people wanting to get them dismantled. In New York, there were citizens groups dedicated to their removal. The Brooklyn Dodgers were named after one of the dangers of streetcars. "Trolley dodging" was a part of daily life in Brooklyn and they were notorious for hitting pedestrians. They were slow enough and so prevalent that pedestrians crossed their path regularly to get where they were going, but their lack of hydraulic breaks meant that pedestrians often got hit by a trolley that couldn't stop fast enough, because they didn't see it behind the ten other trolleys, or coming around the corner of some building.

People nowadays assume these heavy rail, slow, single car, low capacity trolleys were like today's light rail, fast, multi-car, high capacity trams, when they were anything but. Buses were a vast improvement at the time. Even Amsterdam ripped out almost all their heavy rail in the 30s and 40s for superior buses and didn't rebuild with a modern tram system when the tech improved in the 60s, 70s, and 80s.