Avie was one of the lead developers of Mach at Carnegie Mellon as well, so his legacy extends beyond macOS and NEXTSTEP.
But I'd argue that beyond code, probably his most important Apple role was establishing a steady release schedule. After the disasters of the Pink (later spun off as Taligent), Copland and Gershwin megaprojects, Avie managed to get Apple to focus on making small steady releases.
And while MacOS X was a megaproject, it too was released in steady steps: Rhapsody, Mac OS X Server 1.0, MacOS X Public Beta and then finally Mac OS X 10.0
