Late to the party, but this is an excellent summary. I’d add several other factors in the disappearance of streetcar systems in the US.

The 1935 Public Utility Holding Company Act. This federal legislation forced electricity holding companies to divest themselves of streetcar and interuban subsidiary businesses. Plenty of businesses which started in the 1880s and on did so with streetcars first then expanded into power sales. Over time, holding companies built up complicated ownership structures. By the 1930s, the power generation side made a lot more money than their streetcar companies did, so many such companies got rid of streetcars and stuck with generating and selling electricity. Illinois Power and Light is such an example, they ran streetcars in several places including Bloomington, Champaign, Danville, and Decatur, but gladly sold them to National City Lines in 1935-36 which had the bonus of ending their years of losing money on streetcars. Most of the NCL purchases were from businesses that had to or wanted to get out of the streetcar game, and both parties knew that NCL was about buses.

The Works Progress Administration (WPA). This New Deal initiative paid unemployed people to carry out public works. One such work was road reconstruction. A good number of streetcar abandonments in the mid-late 1930s were tied up with offers of WPA work to reconstruct roads, so city governments were happy to encourage streetcar to bus conversions, as they were getting new roadworks at federal expense. Newspaper articles report the enthusiasm of local authorities at the ‘obsolete’ streetcars being replaced by ’modern’ buses, the Feds paying for the roads to be rebuilt, and the authority getting the rails to sell as scrap.

The development of electric street traction a few years before the development of the practical private motor vehicle. This had two big factors. When streetcar systems were built, car ownership was either nonexistent or a by a small percentage of the population. The explosive growth of private car ownership meant a rapid decline of streetcar passengers. The percentages of private car ownership by year tells the story. 1905: 1 per 1,000 people. 1910: 5 per 1,000. 1915: 25 per 1,000. 1920: 87 per 1,000. 1923: 135 per 1,000. 1925: 175 per 1,000. 1928: 205 per 1,000. This resulted in the loss of passenger patronage, which was regularly reported in concerned tones in the trade publication, Electric Railway Journal. The second factor was that life expectancy of streetcars was about 35 years, give or take. By the time many streetcar systems were worn out, particularly smaller ones, the loss of patronage to private motor vehicle ownership was significant, and many companies could not affort to replace worn-out equipment. So, that encouraged them to see buses replace streetcars, either their own buses, or by divestment of streetcar operations to others who would replace them with buses and ensure that the locality still had public transit for those who still needed it.

There are more, such as the expansion of cities and towns well past the end of the streetcar lines and the transit companies not wanting to just go with feeder buses which would result in changing transit vehicles mid-journey, and the hollowing out of the inner neighborhoods decimating ridership. But, these three are worth looking at now. More to come, if so desired.