Yeah, I think "Super Smash Bros Ultimate" will be the final game. But I don't think the Nintendo Switch version of Super Smash Bros Ultimate will be the final game. Like others have said, I think they will probably just port it over to the next system. Some people say they would reboot the series without Sakurai's help, but to me, as long as Sakurai is alive he will be helping on the games, if they were made. He's admitted to being a sort of control freak, and has to oversee everything in development. Plus to me, a reboot doesn't make sense. People almost long their minds when the Ice Climbers and Wolf were cut in between Brawl and 4. Imagine how people would react if they cut the roster down to even 25 characters. Newcomers like K. Rool, Ridley, and Belmont would likely not have a place on that roster, because they're not huge name Nintendo characters. The roster would probably consist of series mascots and main series side characters, like Mario would get Luigi, Peach, and Bowser, Legend of Zelda would get Link, Zelda, Ganondorf... I don't think Sakurai would be willing to cut those fan favorite characters mentioned above after waiting so long to add them in.
Personally. even as a HUGE Smash fan, I would be fine with this being the last game. The series should go out on top, and I'm not even thinking about the next Smash, but rather fully ready to enjoy this new game.