The reason that fans are making that assumption at this time is reportedly because Tekken 7 will have all of the downloadable content characters, patches and gameplay changes that have been released into arcades in Japan over the last year. Another reason to speculate just that is the fact that Tekken 5 and Tekken 6 were based on the arcade versions as well.
The new game will have twenty-nine playable fighters, with just twenty of them being a part of the default cast. Other characters will be added over time with only one returning character, seven brand new ones and a new form of Jack from Street Fighter.
Video games get pushed back all the time and many times is has to do with the demand of the video game developers to create something much better than before. Developing a game takes time and as the technology improves, the time it takes to develop the games expands making players wait longer in some cases.