Won't even go into the Card side of the ledger because it's a known quantity. As for Ewers: competed and exceled at highest level of TX HSFB as an underclassman, generational arm talent, one of three QBs with perfect composite rating, exudes confidence, "gunslinger" reputation (see point re confidence), strong pocket presence. By first kickoff, will have 3 camps at the NCAA level under his belt, plus hands-on coaching by Sark, et. al. this summer to help further accelerate his familiarity with our system.
Even if he's slightly behind Card on playbook/system, it shouldn't be by much. Ewers is the textbook "high ceiling" play in all other categories. This year is pivotal for Sark and has to know he has to swing for the fences beginning the starting pitch (i.e., Week 1) in general, and on the offensive side specifically given known deficiencies on the other.
And haven't even mentioned the backdrop of how Quinn got to UT in the first place. If there's even a modicum of truth to the Double Eagle NIL war and resulting package, given that and all of the above, I don't see any way Card starts a game this year (absent colossal QE faceplant, or injury).