I don't know why you think they are so committed to him that they would not draft another QB. We already know that they really like Ward and have talked to the Titans about the cost for trading up to take Ward. That price was too high, but it's pretty clear they are not out of the QB market just because they think Rattler has some potential. They also like Sanders, but they think he has a late first or 2nd round grade.
As far as history, they drafted Haener 2 years after they drafted Book and they drafted Rattler the year after they drafted Haener. So they seem to be drafting QBs pretty regularly in the last 3 or 4 years. That's 3 QBs drafted in 4 years.