It was real, but it's not sufficient to explain the mass movement away from streetcars in general. They didn't even purchase a majority of streetcars in the US, and this movement away from streetcars happened in most of the Western world.
US streetcars had real systemic disadvantages mostly stemming from when they showed up in our society; at the turn of the century in uncongested roads when the dollar was strongly tied to gold. This had a couple effects:
* they showed up at the turn of the century, so just in the '40s and '50s as investment started in highways they needed expensive lifecycle replacement. Buses were seen as more flexible and could use this new infrastructure being built with no additional work, so many municipalities willingly switched to buses to take advantage.
* they showed up at the turn of the century, when having a paved road in a city was not a norm, and so contracts allowing for the construction of streetcar lines also expected the streetcar companies to pay maintenance on the paving. Gas taxes usually only pay for major interstate and state roads, but streetcars paid for local streets. Undoing this was not popular since it would require cities to raise new taxes, and would relieve evil streetcar monopolies of a burden.
* they showed up when roads were not terribly congested, so they weren't built with the expectation of needing to be separated from heavy traffic. Cars changed this equation, but dedicating road space to streetcars was just either not on the radar or seen as a giveaway to evil companies.
* they showed up when the dollar was very strong and tied to gold. Laws authorizing streetcar operation also usually involved explicitly tying their operation to a specified flat fare (usually a nickel). This fare was not tied to inflation, and raising the fare or eliminating limits on fares was politically DOA, a tax on the working man and a giveaway to the evil streetcar monopolies.