I think you are missing one thing ... Naruto makes a ton of money for Kishi (ok maybe not a ton but...).... There will be a 3-rd part and a couple of movies... at least 3 (including "The Last").. and this will be purely for MONEY... After that the popularity of the manga can decrease but it's still will have huge fan base.... After all Naruto is the most popular manga in the world.....
One should have thought that if there were to be a 3rd part then Saiyan Island would have said something about it if it was confirmed. Then again, they may just be slow. Though I think a Part 3 is possible, I just doubt that this is the official confirmation of it.
Even though I would like a part 3 , I feel like there might be a final time skip in the ending of the movie showing naruto as a hokage. That would mark the official ending
That we will get Part 3 in some form (be it a manga, anime or just a movie series) was rather clear once the whole Naruto the Last + the Naruto New Era Project was announced. Why else call it New Era Project if Kishi was only going to make a single film as conclusion of Naruto? There will be a spin-off/continuation of the Naruto storyline, we just don't know in which form, just wait the next weeks till the announcement comes or other legit information is available.