Perhaps I am missing something, but I don’t see HomeKit with Siri being a replacement for Alexa. We only rarely use Alexa to turn on/off lights. The home is automated, with motion sensors, Hubitat rules that trigger on device or time events, etc., so the house does what is expected without device commands.
What we do use Alexa for is to recognize phrases such as “It’s bedtime” (which closes drapes, sets light color and intensity, checks barrier devices, turns on alarm, etc., and plays nighttime music), or “wake up the bedroom”, “wake up the kitchen”, etc. Alexa only recognizes the phrase and triggers a virtual switch that is exposed from Hubitat. Most real processing happens on Hubitat.
Yes, I realize I could sprinkle Picos around the house, press this button to wake the bedroom, press another button for bedtime, etc., but that’s not what my wife wants. I don’t see that recognition of arbitrary phrases is something that Siri can do.
The HomePod Mini's use Apple's S5 SoC from one of the Apple Watch Series 4/5 and have 1GB of Ram and 32GB of Flash so they are far, far more capable than any other "smart speaker" on the market.
You can create a short cut with any phrase you want. Then have the action turn on or off something in HomeKit. You can then talk to the HomePod or mini just like Alexa. Just tried it on mine and it worked fine. Made sure my phone was out of earshot so the test would be accurate.
Siri can and does. In fact Siri's voice recognition is better than that of Echo. Time for bed? "Hey Siri, goodnight" and my house is set for sleep time. "Hey Siri, working from home today" and my house goes into work from home mode. And so on...
Who said permanently? Mine are in mounts that allow me to take them out and use them wherever I want to. An iPad is a completely portable device, an Echo Show is not.