Time will tell. One or two of these songs written 70-80 years ago by Rodgers and Hart ("My Funny Valentine", "Bewitched, Bothered and Bewildered"
, George Gershwin ("Someone to Watch Over Me"
, Jerome Kern ("The Way You Look Tonight", "All the Things You Are"
, and Cole Porter ("I Get A Kick Out of You", "I've Got You Under my Skin"
are part of the repertoire of almost every contemporary vocalist today, to be sung and/or recorded. A week does not go by without hearing at least one.
As for full musicals, since Sondheim does not write the book to his musicals, how do you compare "his" musicals to others? Should we say that a show is by "Sondheim and Lapine" as we say Rodgers and Hammerstein? Sondheim and Lapine will most likely be judged in the future by how often the properties are licensed and revived, compared to the shows of others. Some cliques will hold all Sondheim in the highest regard compared to musicals before and after, believing that those who don't "get" the musicals are just of a lower intellect.
Now I will duck and cover. I really am warming up to Sondheim, gradually..