I can see where you are coming from. But you comparing totaly different situations. The first cases got its roots in the behaviour of the players and in Dembeles case of Barcelona too. A behaviour i don't think you want to see from a potential United player. And you see how Dembele performs with that kind of mentality. Mikhi's case was that he broke his own words, driven by Raiola. And i guess some would agreee in here that he is not a nice person and you are fecked if you trusted him.
A potential transfer from a well runned club isn't complete in 2 weeks, cause they would have solved it a lot earlier. Take all the other examples at other clubs, its done or nearly done, cause you have to be quick, and don't identitfy targets within 2 weeks, you did it long before, and try to get this done early. Which i guess whas missed from United. Therefor a clear deadline, in the public. Something which is quite unusual for BVB. But it seems that even that deadline was missed from United. Now with all the strong statements of Zorc, Watzke and Favre, it would be hilarious for the board to ruin their reputation and sell after that deadline (besides the already mentioned fact that there was a reason for the deadline: playing the best possible season).
And you are missing the financial point of view: you got an asset with a long term contract, what is worth a lot, but in the moment nobody can pay that value. So of course you uses it once more, get your season targets done, and relax for coming bids next season. You would literaly lose money if you sell now. They would have probably sold cause they valued it with 120m, that would have made sense to them,with one or 2 successors in mind in the right timespan. But this is over so you just let him play one more season. This perspective shouldn't be hard to understand. I understand your point of view, but the contract situation is better, the players mind is better and his advisers too(something you should be glad about if you want him), BVB position is better, the market was better, there was time for buying new players.