Historical data shows that about 50% of QBs drafted top 5 pan out. QBs selected out of the top 5 in round 1 since 2000 have a 15% chance (4/26) of panning out, which has been seriously upped by the emergence of Mahomes and Watson. Prior to Mahomes and Watson the numbers were 8% (2/24). When you drop down into round 2 or later, its about 1 in 30 pan out. The worst thing to do is look at 2017 as the standard of expectations. 2017 was a severe outlier when it comes to QB drafts.
2003 Leftwich, Boller, Grossman
2004 Roethlisberger, Losman
2005 Rodgers, Campbell
2006 Leinart, Cutler
2011 Locker, Gabbert, Ponder
2012 Tannehill, Weeden
2014 Manziel, Bridgewater
2017 Mahomes, Watson
This also assumes Mahomes and/or Watson aren't one year wonders and do not collapse next season.
If you want to find a QB, the best approach is to keep drafting them in the top 5.