I don’t think the integration is poor.  The speakers are voice-enabled speakers not Echo devices (or Google devices).  I think some people’s expectations were out-of-line with reality.  That apart, I do agree with a lot of what you say.  I bought Echo Dots because I already had speakers that were not voice-enabled, but I would not swap my Play:1 + Dot for a Sonos One.  As well as the reasons you give, I like the freedom to place the Dot where it is best for receiving commands, which is not necessarily where the speaker is.

I assume you have used Groups in the Alexa app to make the music come from the Sonos speaker rather than the Dot?

I agree with this recommendation, but I don’t fully blame Sonos for the limitations of the integration.  Some of the limitations are all that Amazon allows, while others surely get relegated as lower priorities for what is primarily an audio company, not a voice assistant company. Sonos would be forever trying to add features to catch up with whatever the latest Alexa or Google Home feature while also trying to make a better speaker than other audio companies.

The other reason is that it’s just logical to separate voice assistant functionality from speaker functionality. For one, mics work best when they are closer to the person speaking, while speakers need to be a certain distant away from the person.  For two, it makes sense to be able to update your voice assistant hardware without having to upgrade perfectly working speaker hardware.  All-in-one devices are nice in some instances, but that have limitations.

If all you want from Alexa or Google is music control and some basic smart home control, than Sonos is just fine. I still use it in a couple bedrooms in a bathroom.  For every other room in the house, I use an echo show or dot.