The integration with Alexa and Spotify is poor, probably because Amazon would prefer if you used their streaming service (which is not as good as Spotify)
Nobody from Amazon really looks at these pages with a view of doing anything about the many grumbles. Most of the replies are from other users of the Echo devices. The mods (Amazon Staff) are here to keep the peace and help out where they can. You will need to send feedback to amazon using this link https://www.amazon.co.uk/gp/help/rsvp/leave-feedback.html
A clunky workaround would be to convert your Daily Mix to a play list using the Spotify App or Spotify program on your PC
Yes, I used to be with Music Unlimited when I first got my Spot, because it was the only option. I did contact Amazon with feedback, comparing their service to Spotify, but never heard anything back, and nothing changed while I was using the service.
When my sub expired (and now that I can use Spotify with Alexa) I changed over. BTW I did send feedback via the Alexa app on my iPad about this too.
It's not the end of the world, but it would've been nice to make full use of what Spotify provides.