I guess that depends on what your own self-image and self-belief is: are you THE star or not? If you believe you are the best at what you are doing and the metrics back it, then how threatened should you be? Maybe the new boss and his "star" set the bar really low (like competing FCS), so its not even really a comparison.
That being said, I think your analogy is a little flawed. It would be closer to "new boss brings in star talent AND a new process - one that is similar to yours but is going to take some learning. NOW, do you have the skill set to be a good learner AND excel at the job?"
I hear your points, and ultimately, any QB who wants to play in P5 must beat out talented competitors. JdL will have competition at Arizona, and he'll have to learn a new system there, too. He won't be able to run over to the sideline to get plays like he did in 2020 with Rolo, and some of his competitors there already have learned and worked through Fisch's system. More generally, he certainly will need to compete if he ever is going to cash an NFL paycheck. To have a shot at that, though, everyone other than Matt Cassell needs to play.
As for that analogy I gave, while no analogy from normal careers can be perfectly analogous to QBs competing for PT, there are reasons to view JdL as even more rational in looking elsewhere than someone in that hypo I gave.
First, I didn't want to make it a novel so I deleted this, but unlike everyday people who might stick around and give things a shot (even with the new boss and the hotshot rival), able to leave anytime they can find something they like better and start there immediately, it's possible in a QB situation to not win the job and just never get on the field, despite doing pretty well on the practice field as the QB continues to learn and develop, maybe even better than the guy who wins the job.
Also, unlike the everyday worker who can give things a go and bail at any time, realistically, these QBs need to make moves at certain times or be stuck losing a year of precious eligibility. Joe Worker might just "waste" months in a situation with little chance of coming out on top, but he's still getting paid. JdL, if he sticks around despite knowing the uphill battle he might face with Morris/Ward, risks losing a whole year of experience and getting tape and exposure.
Third, this is consistent with your point, not arguing against it, but not only has the new OC here brought in his guy, he has a new system. Not only would JdL have to learn it and work to excel in it, but the guy he is competing against already has (albeit at a lower level).
Anyway, the main reason I brought this up was to get people to try to put themselves in his shoes and consider the downsides. It's easy to say somebody shouldn't be a ____ and should compete. That's my reflexive thought, too. Nobody wants players to bail when facing competition, whether a result of growing up in a culture of daddy ball and participation trophies or something else. But when you're the one facing the downside of a situation that doesn't go your way, there's more at stake.