WHY DOES IT NEED A NEW MODEL.
This thing is a home speaker. It should be able to be sold for 5 years, hell even a decade, without requiring any updates.
One reason might be to optimise (reduce) the production cost maybe with a view to competing directly with the Sonos Era 100 (US RRP $249 I think and often on sale in the $179 - $199 price range vs US RRP of $299 for HomePod 2).
Older components are not always cheaper, in fact once you factor in dwindling production runs there is often a crossover point where they become more expensive than something more up to date due to economies of scale and other logistical efficiencies so it's at least conceivable that Apple might be able to get the RRP of a HomePod 3 down to $249 without any reduction in quality, or ideally still with an improvement in audio performance due to more powerful audio processing hardware.
There were rumours swirling around quite a while ago that Apple was looking to buy Sonos. I think that ship has now sailed but I sometimes wonder whether that was a missed opportunity because Apple could, and in my opinion should, do more to bulk out its audio offerings both in terms of range of products and pushing at lease a bit higher up in terms of audio quality with some extra products.
Stuff like soundbar(s?) potentially able to be augmented by existing or next-gen HomePod devices, next generation AirPods Max, ability to use HomePod Mini and maybe even full-sized HomePods as rear speakers, a subwoofer (and maybe also a mini subwoofer designed to go with HomePod Mini) would all in my opinion be potentially interesting products.
A Sonos acquisition would have given Apple a real engineering head start for all of those potential products listed above, and quite a few of them could have been stand-alone propositions without needing a buyer to already be in the Apple ecosystem, but I doubt that Apple is ever going to get that ambitious in the audio arena now.