You addressed storage but skipped the CPU portion: Yes, Opus can create good quality audio with low storage requirements, but it cannot do so without a high CPU cost.
The HomePod for example uses the Apple A8 chip which is a very capable chip used to power the iPhone 6 than did way more than encode/decode audio.
Interesting point, my guess would be that not many would notice since a HomePod/Alexa/Google Home would usually sit somewhere in a corner of a room/under the TV and not be regularly touched since you don't need to touch it to control it most of the time.
I am not even sure it would be that much heat, my x86 laptop can play video for a very long amount of time before getting noticeably hot, granted with a fan, however these ARM CPUs get noticeably less hot than your average Intel chip, even without a fan.
Even for the cheaper devices, the CPU is probably capable enough, (maybe excluding the cheapest Echo Dot/Nest).
The Echo Show devices even have a screen and are actually designed to play videos from all kind of sources, (decode), as well as for videocalling, (encode) and they're £60 right now on Amazon UK.