Think of the HomePod upgrade cycle like the AppleTV rather than an iPhone. The HomePod's hardware is fantastic. It was meticulously designed to have the best possible sound Apple could make in such a small package and with microphones that could hear you speak in natural voice from across the room even while the loud music is playing. I didn't expect an updated HomePod for several years. Instead, Apple will improve it through software.
I do expect the entire Beats lineup to be updated to include AirPlay 2 and maybe even Hey Siri. You'll probably get a low cost Beats Pill with Hey Siri before Apple releases a low cost HomePod.