OK, here we have a definition of "legacy".

"something that is a part of your history or that remains from an earlier time"

Now, if am right, and the Zoo song is the only song of Tom's that will survive in the public's consciousness for a long time, then it will be his legacy.
His name will appear along side the title of the song in song books.
When, played on the radio ( or future equivalent) a dj/presenter will now and then name the composer of the song

Let's take another example,

Irving Berlin is a giant of the popular song BUT a hundred years from now, I would bet that, of all his countless songs, only "White Christmas" ( if we are still celebrating Christmas) will be remembered in a big way.

For the general public, it will be all that is left of his great music legacy

Of course, there will always be a revival of interest in songs from an earlier time.

"The Entertainer" was amazingly plucked from a dusty obscurity when used in "The Sting" and gave birth to a huge revival of interest in Scott Joplin and ragtime music.

Tom's music could be "rediscovered" a hundred years after his death.