Nothing about the word niche indicates decline. Video game consoles are, however, a narrow market. Not to say it can't be expanded, but it is niche. Especially when you take it as part of its product intention of entertainment.
People need to look up the term because it is being used incorrectly niche isn't just a segment of a mainstream market but one of specialisation.
We really seem to be downplaying the business, gaming is now larger than the movie and music businesses combined and consoles make up 25% of that business.