Mayock & Gruden both are aware of Carr's ceiling. "He's a good QB, who could be great" aka he is not great, only good. Thats from Gruden.
The issue isnt Carr's ceiling its the Las Vegas Raiders floor. You can expect that the team and maybe even Carr will be improved with more weapons on offense and a better defense. If this team is close to a playoff team now with Carr I seriously doubt they try and tinker with a stop gap. Carr year 3 with WR weapons and a better defense should technically be better. No crazy road schedule, no over seas game and a brand new stadium hype.
Its smart to keep Carr and the status quo moving forward (almost a playoff team) - Doesn't mean you dont do everything in your power to get your hand on a franchise rookie QB contract. If you move up for Tua and redshirt him, "Camp Carr" will get over it. Worse case he plays great and you have trade chips.
The Raiders still need to build the roster out for any QB to succeed. So Carr isnt the only issue right now. He may be a longterm issue but we have UDFA players starting and rookies playing everywhere. This team is still a work in progress roster wise. It would feel dishonest to say Carr is the only thing keeping us from a Super Bowl. Just as dishonest as thinking he will ever be elite.
TLDR: Raiders want to upgrade Carr. Not replace him randomly. I only expect a rookie QB to be added.