Siri

And this requires a Sonos product with an available line-in. My Play:5 is using line-in for my turntable - so getting Alexa to control my Sonos would cost me another $550 - I'd much rather have Sonos support Siri, or even Hey Google.

Nope. As announced back in August, Echo/Alexa will be fully integrated with Sonos early 2017. Private beta should be starting soon:

http://press-us.sonos.com/134980-sonos-with-partners-and-industry-leaders-ushers-in-new-era-of-connected-home-listening

I would assume it's a non-trivial coding task. However, once it's rolled out for the Amazon devices, I would hope they could move on to Apple and Google, since the hardest part would already be figured out.

It feels as though the strength of Sonos is the Industrial Design. But it does not seem like they have a lot of support when it comes to the digital side.

On my end after playing with the Play:5 the IxD and UI are poor for both the IOS and the OSX which is a sign that they do not have an agile approach to development nor the right design insights.

My recommendation > hire people like frog, Method or IDEO to flush out these IxD issues and pivot the product from being technology driven to being customer driven.

Then implement the redefined IxD using the design agencies design research and concepts. Adjust the ID's missing tech requirements (for example if there needs to be an NFC for HomeKit or not, or Bluetooth combination) and upgrade the Apps fast to work with Apple and Alexa and other things.

I shan't argue with you, since I like the idea. But then again, I'm not Sonos, nor am I looking at their budgetary/time/resource restrictions, either. In fact, I've no idea what those are, but they're likely a restricting factor.