I think that you presume that Maye won't be ready until late in the season.
Consider if the team is planning for him to start Game 5 and is prepared to start him earlier or bring in a veteran AT THE TIME if Brissett is injured.
Should we really sign another veteran just in case Brissette gets injured before the season starts? Or should we just continue with Zappe until the final cut, expecting to put him or some other veteran on the Practice Squad. The issue in signing a veteran now is that lots, lots more will be available after final cut.
HOWEVER, if Brissett has a long-term injury before Game One, the team will re-assess, and likely bring in a veteran immediately instead of waiting.