Conversely, I use 'send to kindle' from Calibre all the time and absolutely love how easy it is to send a book to any of the 3 kindles in the house. I just send it and the book is there a few minutes later. The only time I've ever run into any issues with it is when I was loading up a Kindle for a kid with a TON of books and it wasn't happy about so many emails.
Overall though, I agree with your message: you're not really forced into using the Amazon ecosystem at all if you're willing to put in a tiny amount of work and the Kindle's are either sold at a loss or at such a small markup that it might as well be one that it's difficult for me to consider the competition since they cost so much more.