[bc] Bach, Haydn, Mozart, Beethoven, Verdi, they were popular entertainers, yes?
You can't actually lump them together. If this were a quiz show, which one would not belong? The answer is Bach.
Handel was quite popular in his London corner of the world, though the vogue for Italian opera eventually faded. Or perhaps it was his style of opera the English had lost interest in. Pergolesi had come on the scene, after all, and life had gone on.
Bach was a small man in a small corner of the world, insignificant, overshadowed by his children, first made truly famous by Felix Mendelssohn in the 1820's.
Haydn was a medium level celebrity, I suppose, in his obscure Esterhazy country home. He entertained his master's guests and published in Vienna, but was not allowed to leave town until the next generation came to power. When his new master/owner was no longer interested in his music, he was allowed to travel and become popular, again in London, of all places.
Mozart made his living off of music without selling his soul to a master as Haydn and his own father did. His father thought this was all a catastrophe and that Wolfgang was making a terrible mistake. To earn your living from the public means you must adopt a popular perspective. Mozart was multi-talented and could sell his skills in a variety of ways--as a composer, as a pianist, as a violinist, as a teacher of piano, as a teacher of composition....
By Beethoven this style of living off the public was no longer thought shocking. He was a notorious madman found sleeping in the snow and ridiculed by Goethe. In short Beethoven was a true celebrity.
Verdi was popular on a different scale entirely. He is virtually, though not literally, considered the father of his country.
The phenomenon of celebrity grew with the passage of time. But I think, bc, this wasn't what you wanted me to write about. Hummel, Paganini and Liszt were much bigger celebrities than your list, but the music they were writing and performing was in the same basic popular style as that of their now more highly regarded colleagues. The split between pop and classical did not exist.
Wagner singled out other composers for systematic ridicule, it's true, and placed himself on an incredibly high pedestal, but in general the writing and performing of kitsch for public consumption was just considered a necessity, not an indication of inferiority.
It is a great tragedy that Leonard Bernstein was made to feel that the music he wrote for Broadway was not good enough, that he had to abandon it for "serious" works. The only thing that will revive the popularity of contemporary opera is for composers to get together and decide that it's ok if the general public actually likes your music. If, perish the thought, someone should actually exit the theater humming one of your tunes....
Pardon me. I have started to rant.