I dare not speak for JRiver, so this is my opinion and guess on this subject. I think part of the issue here is not everybody is using Alexa, I sure don't own any Alexa-powered devices, I have several Google Assistant-powered devices. If JRiver decided to start going down the smart assistant rabbit hole by adding Alexa support, then they'd have users asking for Google Assistant support, Siri support, Bixby support, Cortana support, etc. At least with Alexa and Google Assistant, those are probably the two most popular ones (Siri could be considered as much too, but since it's Apple there's probably extra hoops involved). It sounds like it's a deep, time consuming (and thus costly) rabbit hole to go down and who knows what hoops the manufacturers have in place that have to be jumped through when adding support for various smart assistants. I'd also wager the majority of MC's users, myself included, don't actually care about support for smart assistants.
And that's not even considering things like the potential privacy issues adding support for the various smart assistants could open. Honestly, in my opinion sometimes it's best to leave it to third-parties in that regard - I know that's not what everyone wants to hear and it sucks that House Band went away, but remember that in software development not everything everyone wants can be implemented and this could be one of those things. It simply may not be worth the development investment currently by going down that rabbit hole. Of course, I could always be wrong too, but this is my opinion on the subject.
