Your point about the USC-BYU game shows either you still don't know much about PAC fan bases, or you are being intentionally misleading.
USC has a great following and fan base when they are competitive and winning. When they have years like 2021, the fan base checks out and waits for next year.
BYU vs. USU last year, 1.26M, outdrew Utah vs. ASU, 1.21M, Utah vs. Stanford, 1.04M, Utah vs. Wazzu, 470K, and Utah vs. SDSU, 326K.
Those numbers right above are a direct reflection of what Utah draws when they are the big name in the matchup.