It comes down to two things - money and what Rush wants.
He's a free agent after this season, so Jerry will have to pay him if he wants to keep him, which would seem to be an obvious decision, he's done better than any of the other Dallas quarterbacks - Roger, Troy, Don, Tony, Craig or whoever - won his first 5 starts. That's valuable, can't ask more of a backup (though other backups have won the SB), nice that he's over achieved. With what he's accomplished, other teams will be hot after him, if not for a starting job then for his back up abilities. So, does Rush want big bucks? A starting job? Both?
If he's as smart as he appears to be, he'll stick around with a team he's familiar with, with a good young defense and take whatever Jerry offers him (if it's a decent offer) and be able to make good money while being under no pressure to take Dak's job. Pretty sure he understands he's not really NFL starting quarterback caliber, why risk being a starter somewhere else and not being as successful as he is at Dallas?
But as a side note, saying he won't be considered as a starter somewhere else isn't for sure, we've seen some NFL owners make some really stupid player decisions, wouldn't be the first time!