There's a couple of actual flaws with the tank plan that you're ignoring too. First off, Bill is chasing Shula and he's not considering a tank. He's putting his own individual goals above the team, which seems to be a trend in the last 5 seasons, while before it was team first. With his steadfast refusal to do a tank, he's actually setting the team further back because tanking for a high draft pick impacts one season, while ripping off two 7-9 seasons in two of the last 3 years is worse than a tank because the draft pick is middling and not high enough to get a true impact player unless they got lucky. Which they have not.
Second off, even if they did a tank and got the top pick in the draft, Bill views QBs as a dime a dozen. He's always claimed he can win with a middle of the pack type of game manager, so him using high end draft capital on a guy like Lawrence probably wouldn't have been in his plans even if he had that pick at that time. Again, this is him putting his own goals ahead of the team, and just another glaring example of why he's creating quite a lot of the problems and issues the team has on his own. Just like not calling around to maximize the trade for Jimmy G back in the day, forcing Brady out because he wouldn't commit to a 2 year below market value deal, thinking Cam Newton was a good call, having his son run the defense, putting Patricia and Judge in charge of the offense, reaching for every pick they made in last year's draft, etc.
It goes on and on, but it all stems from Bill and his stubbornness.