In the years I have been involved with the D community, I have watched it grow from a handful of programmers posting on an obscure newsgroup to a large, vibrant community of users, contributors, and an annual conference. The language has changed and improved dramatically over that same period. As someone who enjoys using the language and who keeps abreast of D-related news and discussions in several different online communities, I'm quite optimistic about D's staying power.
On August 24, 2015, Andrei Alexandrescu announced in the D forums that he was resigning from his position as a researcher at Facebook to work full time on "pushing D forward." On October 16, 2015, he announced the incorporation of a D Foundation to promote and develop the D programming language. One of D's greatest strengths is that anyone in the community can contribute toward its development, but that has also been one of its greatest weaknesses; contributors tend to focus on the aspects of the language and its ecosystem that they enjoy or need themselves. This has led to a number of holes that contributors have not yet filled. The establishment of a D Foundation is expected to help alleviate that situation.
Another optimistic sign is that D is being used now by companies in production, including in scenarios where high-performance is a critical requirement. The success of these companies has proven that D is a serious contender in different arenas. It has also resulted in some direct contributions back to the D community, such as the containers library released by EMSI. As more companies find success with D, more will begin to adopt it, ultimately benefitting all D users.
It is my hope that you find the same enjoyment in the language that I have found, and go on to develop new software with D, whether open source or closed. I've always viewed the D ecosystem much like the Wild West; there are so many unexplored possibilities out there that anything goes. If you have an itch that existing D tools and libraries can't scratch, I encourage you to put your new D skills to use and develop something that does. I also hope that this book provides some help along the way.