We prove the longstanding physics conjecture that there exists a unique two-parameter
${\mathcal {W}}_{\infty }$-algebra which is freely generated of type
${\mathcal {W}}(2,3,\ldots )$, and generated by the weights
$2$ and
$3$ fields. Subject to some mild constraints, all vertex algebras of type
${\mathcal {W}}(2,3,\ldots , N)$ for some
$N$ can be obtained as quotients of this universal algebra. As an application, we show that for
$n\geq 3$, the structure constants for the principal
${\mathcal {W}}$-algebras
${\mathcal {W}}^k({\mathfrak s}{\mathfrak l}_n, f_{\text {prin}})$ are rational functions of
$k$ and
$n$, and we classify all coincidences among the simple quotients
${\mathcal {W}}_k({\mathfrak s}{\mathfrak l}_n, f_{\text {prin}})$ for
$n\geq 2$. We also obtain many new coincidences between
${\mathcal {W}}_k({\mathfrak s}{\mathfrak l}_n, f_{\text {prin}})$ and other vertex algebras of type
${\mathcal {W}}(2,3,\ldots , N)$ which arise as cosets of affine vertex algebras or nonprincipal
${\mathcal {W}}$-algebras.