I don't necessarily disagree with you on two things -- that you can get a franchise QB without tanking, and that success rate is low (since 1999 -- not counting this season --
only 28 of 74 have been "franchise" QBs). Both can be true. But it is also true that we shouldn't discount that most QBs in the NFL that find success end up being high picks.
Just look at the breakdown of current NFL QBs:
1st round picks: L. Jackson, J. Allen, J. Burrow, J. Winson (D. Watson), B. Nix, A. Richardson (J. Flacco), T. Lawerence, P. Mahomes, J. Herbert, D. Maye, A. Rodgers, W. Levis, K. Murray, C. Williams, J. Goff, J. Love, M. Stafford, S. Darnold (JJ McCarthy), D. Jones, B. Mayfield, J. Daniels (Mariota 1st), CJ Stroud, T. Tagovailoa (potentially back next week according to sources).
23 of 32 teams have starting 1st round QBs.
2nd round picks: J. Hurts, G. Smith, A. Dalton (B. Young was 1st), Derek Carr (Rattler 5th)
27 of 32 teams have 1st/2nd round starting QBs.
3rd round picks: R. Wilson (J. Fields was 1st)
4th round picks: K. Cousins (Penix is 1st) , D. Prescott
30 of 32 teams have 1st-4th round stating QBs.
6th: G. Minshew
7th: B. Purdy
...There are a number of high draft picks in there as well as backups beyond the ones I listed in parentheses. You can call it user bias that all these guys are still in the league, but it's a reality -- most starting QBs in the NFL have to come from the 1st round. Rare is the Tom Brady.