The reasons for discontinuing Watch Later and plug-ins are discussed in this announcement, and the blog post that is mentioned there.
As for trailers, there are feature requests dating back several years asking for trailer support. I don’t disagree that it should not be on by default, but turning trailers on/off is a simple switch, accessible when you edit your Movie Library.
Each client app also has a switch to enable/disable Trailers. The server-side Library setting above and the client-side option must both be enabled in order for trailers to play.
The settings that you find in Server Settings regarding Trailers only control which trailers are shown.
