In a conversation with Deadline, Mendes says that he's completed the Bond story that he wanted to tell...
"Without giving too much away, the difference here for me is, this movie draws together all four of Daniel’s movies into one final story, and he completes a journey. That wasn’t the case last time. There is a sense of completeness that wasn’t there at the end of 'Skyfall,' and that’s what makes this feel different. It feels like there’s a rightness to it, that I have finished a journey."
So with Mendes moving on the search for a new director will probably begin in earnest. But what about star, Daniel Craig? There have been just as many questions about his future and if this is will be his final turn as 007. There have been some conflicting reports about his status; is he still under contract for one more movie? That's been the prevailing idea, and Mendes doesn't dispute that when asked about it by Deadline. But producer Michael G. Wilson recently told THR they "don't have a contract" with Craig for a fifth movie although he's confident in a return. There may also need to be a new studio as Sony's deal to distribute Bond films is now finished.
The Bond franchise heads into uncertain territory at a time when they'll need to start gearing up for the 25th movie, which will obviously be a huge deal.