1. IT professionals absolutely do use such systems. In droves. It's true that we are more likely to be wary of them, but a large number of IT pros I know and work with do use Alexa/Siri/etc. for home automation and all kinds of other things. And no: smart speakers are not "inferior to any type of input mode", at least not in a wide variety of circumstances. I can turn my air conditioner on or adjust its temperature while lying in bed, without getting up, and without even waking up all the way. Try that with another "type of input mode". I can turn on lights while I am stumbling to the bathroom in the middle of the night, but I can easily use only 10% brightness so as to not wake up my wife. Try that with another input mode. I could go on for pages, but I trust the point is made.
2. There are at least 300 million smart speakers out there already, and the number is growing exponentially. Amazon alone already had 100 million sold by the beginning of 2019. I don't know the exact number right now, but it's a very big number.
3. See #2. 300 million units is not a "niche".
4. Brand loyalty, in fact, gives substantial benefits to the consumer, contrary to your assertion. Greater integration of devices and ease of use is the most obvious one.