It was not just the ballets Stravinsky composed that is important for Balanchine, but the fact that it led to Balanchine's creative impulse being inspired to use Stravinsky's music in total for some 35 works(including reworkings of earlier works.) This I believe makes Stravinsky historically more significant to Balanchine oeuvre than Tchaikovsky although I am not sure how many works Balanchine choreographed to Tchaikovsky but surely not many more than ten.
The ballet audiences began to move on from Tchaikovsky a hundred years ago with the advent of Diaghilev. Alongside concert hall and opera house performances, Balanchine probably helped hundreds of thousands of people to know Stravinsky’s music opening them towards appreciation of more modern music. Since then all kinds of ballet and dance companies across the world today offer Stravinsky regularly, as well as what one might have once described as less accessible “avant garde” music far removed from Tchaikovsky.
By the time Balanchine created his successful “Mozartiana and “Serenade” ballets to Tchaikovsky’s music, he had already been choreographing for 13 years and possibly already created 50 works including “Ragtime”(There were several later versions) , “Le Chant du Rossignol” and the masterwork “Apollon Musagete” all to Stravinsky’s music.
Balanchine’s Tchaikovsky ballet, are for me sublime and uplifting and yes and they were in the past easier on the ear than Stravinsky for some members of ballet audiences. But I would suggest that Balanchine did not choreograph just for a popular audience, but followed his artistic inspiration and Stravinsky was tied deeply to his conceptual process. The thirty five works to his music confirms this.
You state, "I think ballet would be exactly where it is now if Stravinsky had never written for it, but not if Tchaikovsky hadn't." Possibly that is the case for ballet in general, but I am not sure it would be true for New York City Ballet or for its repertoires great influence around the world.
PS Balanchine had staged revised versions of elements of
Tchaikovsky’s ballets in the 1920’s.
Balanchine was certainly a great choreographer and a charismatic person (by all accounts) and you are right, NYC would not be where it is today without him, but on a global scale there were others who had just as much, if not more, influence on trends in ballet than he. I love his ballets, but I think there is a danger in idolising one person to the exclusion of others. In some ways he was very narrow-minded in his 'vision'. I think Fokine and Ashton had a greater influence overall on 20th century ballet.