The Top 40 has been an effective way to showcase popular music for decades. The Top 40 could be considered a marketing gold mine for many record labels. The songs on this list are played often on pop radio stations. This great exposure to large audiences helps boost record sales for artist that enjoy a spot on the weekly Top 40.
Yes, the top 40 is effective in showcasing popular music played on the radio. In my opinion, the top 40 focuses on the most popular songs played on the radio. In particular it seems to focus on the most popular songs on the most popular radio stations. While I'm sure there are many niches of people, genres, and songs that aren't acknowledged on the top 40, what the top 40 does showcase is the most popular songs, for the largest group of listeners.
Yes, top 40 demonstrates current trends in popular music. The top 40 is essentially the most played music on the radio. It is music that appeals to and is supported by the masses. It doesn't say much about actual musical talent or innovation, but it is what the general population wants to listen to at any given time.
No, the weekly top 40 is no longer effective in showcasing popular music. It was an effective medium in previous decades, but now, people can listen to music via their cellphones using Spotify or a whole variety of different platforms. This means that listening to the radio has declined significantly.