Full Idea
Even if set theory's role as a foundation for mathematics turned out to be wholly illusory, it would earn its keep through the calculus it provides for counting infinite sets.
Gist of Idea
If set theory didn't found mathematics, it is still needed to count infinite sets
Source
Michael Potter (Set Theory and Its Philosophy [2004], 03.8)
Book Reference
Potter,Michael: 'Set Theory and Its Philosophy' [OUP 2004], p.51