I think that BYU football will accomplish some nice things in the next decade. I am almost certain that regardless of what happens in 2018 or with Kalani, that BYU football will one day have a few nice seasons and be back in the Top 25. We have a big program, with a solid recruiting base, and with nice facilities.
I am really unsure what to think about our program right now. I am encouraged by Grimes and his staff, but a new staff usually takes half-a-season to gel. The defense was terrible last season, yet the only coaching change was the coach I liked the most. Injuries were terrible last season and nothing changed with our S&C program. So I don't have much reason to be hopeful for 2018.
I don't think it's all downhill from here (see first paragraph), but I strongly doubt BYU football will ever regain our early-80's level-of-success (66-9 over six seasons capped by the NC). We simply cannot compete with the SEC, the Clemsons, and the Ohios States in today's CFB environment.