1) I'll tell you why. watchOS 3 is a user experience nightmare, unless you like to click, touch, click, touch, click touch, all day long. It was way better to swipe than continually change how you are interacting with the watch.
Swiping is even worse as it doesn't work all the time and there are also differences in how you swipe. Using buttons, the crown for scrolling and tapping to activate something is far more accurate as it works all the time. Not only that, it's also faster because swiping only moves you to the next app so you have to do it 5 times if you want to go from the first app to the 6th. When you use the crown you can do it extremely quickly as it scrolls a lot faster. With a little practice you can do it in 1 movement under less than a second. Not to mention that this all works with a wet/moist finger without any issue. Capacitive touchscreens go haywire when you do that.
The crown is the most used way of controlling the watch in my case. To preview the apps in the dock I click the side button and then scroll with the crown which is near my finger (nearer than the display is). When I click the crown I go back to the watch face (like hitting the esc key) and when I want to interact with the app I simply tap it and then do what I want to do. I like that this also works with the recently opened app (that one also updates just like the apps in the dock).
What I dislike is the fact that you actually have to swipe to go from one watch face to the other after you've force touched the current watch face. You can't use the crown there as you'd expect (the interface looks similar to the dock so you'd expect to be able to scroll with the crown too).
2) Using screens and swipes made most things in watchOS 2 discoverable. Now you need an instruction manual to figure out how to use your watch.
Never read the manual and yet I'm able to use the watch. Probably because it is consistent with how things work on the iPhone, iPod Touch and iPad. Previously you did need to read the manual because watchOS 1 and 2 had its own way of interacting (you needed to know if you have to force touch, tap, swipe, etc.).
3) Siri doesn't work at all. Voice dictation doesn't understand anything. It is so much worse than on watchOS 2 that I have given up even trying to ask Siri anything.
Siri is a PITA in any language that isn't English let alone when things are bilingual. When voice recognition was first introduced somewhere in the 90s it didn't understand when you had things set in Dutch and you wanted it to play an English song. Now it's 2016 and here we are, pronounce something in English and Siri has no clue what you are saying. This is why non-English speakers hate voice recognition/dictation/commands/etc.
Swiping should switch open apps, just like it does on every other device Apple makes.
Not in this world. None of the Apple devices work the way you describe here. On iOS devices you always have had to double click the home button and then swipe between the open apps. You could do it like in OS X when you are using an external keyboard: use cmd-tab and keep pressing it until you get to the right app so no swiping here either*. The way it is done in watchOS 3 is the exact same way as it is done in iOS 10 and earlier (right back to when multitasking was first introduced, I think that would be iOS4).
TL;DR:
iOS: click home button, swipe through apps
watchOS: click side button, swipe or scroll with crown through apps
OS X/macOS: cmd-tab through the apps*
*alternatively there is Mission Control but it only displays
windows that are opened, apps that are hidden (and their windows) or windows that are minimised are not shown. The actual app switcher is cmd-tab and goes through all the
apps (not windows).