You can get a franchise QB without tanking. The success rate for first round QBs, even high pick 1st round QBs is very low. And just because you are competitive doesn't mean that you can't trade up higher to draft that franchise QB or that you can't find him later in the first or in later rounds. The list of first round bust QBs is very long and picking high by tanking doesn't gain you anything on average.
And while 8 or 9 win seasons may not be enough for hardcore fans, casual fans will buy tickets if the games are fun and they team stays competitive. And nobody is going to buy tickets if they aren't competitive. And if this franchise doesn't sell tickets and merchandise for more than a few years, it's going to move.
Tanking and tearing it apart is simply no guarantee that you are going to come back. Many teams that "tanked" never came back. Look at Cleveland. Look at the Jets. Look at Carolina.
And the truth is that this isn't the NBA and NFL teams don't tank for picks. If a team has a bad record it's because they have a bad team and they earned those losses. But it's not because they tried to have a bad team and even bad teams seldom "tank" until the last two games of the season. Even then, most NFL teams don't do it because there is at best a marginal gain for picking a few picks higher.
The "good" news is that with all the injuries this year and DA as head coach, the Saints likely won't need to tank for a higher draft pick. They are going to earn all those losses.