Well there was a period when it looked like the mainstream was going to switch away from Java, but then Android was released. Java is heavily mainstream still. If you're optimizing, it's still a good bet to stick with. But be ready to jump off if Android switches languages away from Java which may pull enough mind share to make Java a 'niche'.

At least, that's my interpretation of the article's logic mixed with what appears mainstream to me. There's a lot of room for subjectivity, but it's difficult to argue that Java/Android isn't an extremely mainstream direction.