Concerning the loss of power of the characters. Here is probably the explanation :
I put together my character 45 as best as I could. And I have to admit, that even with a maximum gear, the power is not there.
Yet my stats are good, and I'm wearing gear that is raid level of the next expansion (BfA). So it doesn't make sense, I should be overpowered.
I thought to myself, it seems that since 9.0 the power is controlled, limited for all characters below the end-game, with apparently an intermediate level at 50.
Then I remembered a very interesting YouTube video
in French that talked about a patent filed by Activision in order to control the difficulty of the game automatically in relation to your ability. In other words, the game will make target impact easier for people who are beginners for example.
In short, the game is totally rigged !