They already are going crazy knowing that Goku will lose. Because he loses! People can be in denial until episode 131 but that’s all the same. Now or later the disappointment will be the same.
And it makes perfect sense that he loses. After all like you very well said he achieved Ultra Instinct, he didn’t mastered it. So it makes perfect sense that Goku loses.
What can really end DBS in a really bad note is if in the end it’s Frieza that somehow catches Jiren off guard and takes him out.
Because news given by Toriyama about the movie coming in December, do point out to Frieza once again being a central character to the movie story. And he has confirmed that it will be a DBS movie. So we can expect it to be the conclusion or a continuation of however the Anime is gonna end.