As someone who advocates tanking at times, I'll chime in here...
Regarding your best case scenario, I'd say your logic was flawed. Your argument was essentially that since the Colts only won 1 Super Bowl with Manning, there's no point. If that were the case, the number one pick wouldn't have the fanfare it does. Number one picks can change organizations; that doesn't mean they will. Tanking and getting the number one pick is simply a means of acquiring more talent for your team. You still need coaches to develop that talent, you still need a GM who can build the roster, etc. In a real "best case scenario" where everything goes perfectly, you tank and get a franchise QB. Not only does that QB stabilize your team, but there are also other positive impacts, like big name free agents wanting to play with him, and maybe even taking a discount to do so.
Let's consider an alternative scenario where a team with a franchise QB (let's say the Colts, since some were saying the Colts should have done last year) decides to tank. When draft time comes around and we have the number one overall pick, we can listen to any of the offers on the table. In 2016, the Rams gave the Titans the 15th, 43rd, 45th, 76th picks, as well as the 5th and 100th picks in the 2017 draft in exchange for the 1st, 113th, and 177th picks in the 2016 draft. Maybe a team would give us an offer like that. If you already have your franchise QB and you still own the number one overall pick, you have tons of options to bring more talent to your team.
At the end of the day, tanking isn't a perfect strategy, but it can help you get up the future of your franchise and bring in a lot of talent.