  We give a new proof of the slope classicality theorem in classical and higher
Coleman theory for modular curves at arbitrary level using the completed
cohomology classes attached to overconvergent modular forms. The latter give an
embedding of the quotient of overconvergent modular forms by classical modular
forms, which is the obstruction space for classicality in either cohomological
degree, into a unitary representation of $\mathrm{GL}_2(\mathbb{Q}_p)$. The
$U_p$ operator becomes a double-coset, and unitarity yields the slope
vanishing.
