Summary...teams that could need a qb in 2020 are TB, Panthers, Broncos, Raiders, Bengals, Colts, and Dolphins. That's SEVEN teams. Will they all need new QB's? Probably not. But it could certainly be 5+!
Could also see potential retirement from NE, Pitt, Saints, Chargers opening up a spot or two.
Any number over 2 and we'll have competition for Mariota. ANY single team that doesn't pick high would be competition for Mariota (since they won't be in rebuilding mode).
As a backup, Tannehill could have a dozen or more suitors so he'll have job offers elsewhere for backup money he could take over our situation.
Details if you want to read them...
The problem with your theory is this all looks fine on paper when you remove injuries and the human element out of it. But...
What if Brady wins another SB and decides to retire and they don't think Stiddam is ready? Possible destination.
What if Josh Allen doesn't play well this season and they decide to bring in a Vet for competition. Didn't we just do that? possible landing spot for Marcus or Tannehil as a backup.
What if Darnold doesn't play well after he comes back from mono? The HC is gonna be under a lot of pressure to get W's...same as Buffalo.
Keep in mind, since these guys ARE on new rookie deals, teams aren't glued to them financially and can bring in a vet with starter money without wrecking their cap.
Dolphins likely take a top 5 guy...but also could want a vet backup like Tannehill to tutor and or start in case the rook isn't ready.
Texans may want a vet backup and Watson has been very banged up. Could need a backup next year.
Jags need a vet backup now! Could really need one in the off-season!!!
Colts don't look like a team that's bad enough to pick high enough for a qb. Definitely a possible landing spot for a starting QB and or a backup!
Big Ben is signed thru 2021, but has also talked about retiring for the last 3-4 years. If they have a down year due to AB and Lev Bell being gone...maybe he takes his money and retires. Would he want to stick around for a rebuild?
Lamar Jackson looked awesome in week 1 but hasn't it been an NFL rule that you can't survive at QB having a run first qb? High injury risk!
Bengals - Dalton will be in his last year. Do they want him to be a $17mil+ cap hit next year? If they cut him, no dead money and $17+mil cap space! Do they keep him? Probably not. Good chance he gets cut and they go in another direction. If they pick high enough, they draft a qb. If not, probably looking for a new starter and maybe draft someone in round 2-4 to develop while they hope for the best. Potential landing spot.
Raiders - definitely a potential landing spot. Carr has been overpaid for a while and hasn't looked good in recent years. If he doesn't play well, the Raiders could free up $16+mil by cutting him and signing someone else and/or let Mayock/Gruden draft a new guy! Potential landing spot.
Broncos likely to take a high pick or sign a new vet starter. potential landing spot.
Chargers -Rivers is a free agent. Time to move on? Potential landing spot as they won't pick high enough to draft a guy to start next year and if Rivers doesn't come back...potential landing spot.
Eagles and Dallas are definitely backup QB landing spots.
Saints are potential landing spot if Brees retires. No idea what Bridgwater is gonna be yet so they'd be in play if Brees goes.
Panthers are definitely a realistic possibility for a backup and a vet starter! Cam has been hurt and is still playing hurt. His stats have not been what people expect and the Panther save quite a lot of cap room if they cut him! Mariota has a higher career qb rating and a higher qbr the last 3 years. And that's not even taking into account Mariot improving on his own numbers a good bit!
TB - Likely top 5 pick...but also could take a vet qb as backup or to start if they don't pick high or need a vet backup to hold the fort until the rook is ready.