There are many critics of the USC starter Jayden Maiava. What needs to be remembered, though, is that Maiava took over for Miller Moss late in the year. While the expectation at the Division I level is to stay ready and to be able to fill in any role as needed, this is far easier said than done.
Especially when considering the quarterback position, it is significantly difficult to take over for someone under center. (Yes, quarterbacks operate mostly in the shotgun nowadays.)
When considering the communication, rhythm, and timing, there is a lot to stepping in at QB beyond just 'executing better.