No worries mate. Its a forum so we are here to disagree.
I will start with your last point. You say Dortmund are posturing to keep the player. If that was the case then why not simply state it clearly that he will not be sold. Why come up with this hypothetical deadline and a price? The very fact that they did means they are open to selling him.
Yes, I agree that barring a handful, all clubs have to sell their best players, but then that is the point, they do not pretend to be above selling their best players which BVB are doing here (or at least their fans are by claiming they are not a feeder club to the elite). If somebody offered us 200 million for our best player, and that player doesn't wanna go, Utd would reject that bid, because for Utd trophies would be more valuable than that 200 million. BVB would never do that because for them money is important. And this ties into the point regarding club ambitions. BVB are a well run club and sure they have ambitions of staying a top club, but they do not have the ambitions of being THE CLUB in Germany. Yes, this is of course related to their financial realities, which are different from a club like Bayern or Utd, but the point remains that they are not as ambitious as these clubs.
Southampton are widely referred to as the feeder club for Liverpool, which is unfair, but not totally untrue. Soton would accept that as long as they can maintain their PL status with the odd run in Europe and get big fees for their top players, they will be more than happy. That is the extent of their ambitions and they would accept it. BVB fans don't seem to accept the extent of their club's ambitions despite repeated evidence for the same.
And lastly, I do not agree that selling Sancho will see a massive drop in BVB's league position. The likes of Reus, Brandt, Haaland, Hazard etc coupled with a solid defense are more than good enough to get top 4 in the Bundesliga. Similarly, even keeping Sancho I don't see them really winning the league because more than quality, BVB lack the winning mentality that Bayern have in spades.