^^
I gave you two examples in De Gea and Ribery, even Countinho has played well so far and he desperately wants to leave. The only recent exception I can think of is Sanchez but that's because Arsenal are awful. Players tend to change their mind, once they believe in the project again. I really think part of Pogba leaving was due to the Bayern game; reckon he might have thought this was evidence that 2015 was either a fluke or impossible to repeat now that Pirlo, Tevez and Vidal had left, and bought the hype of Mourinho going to Man Utd and bringing Ibrahimovic with him. Pogba would of most likely stayed if he reached the final in 2017, that would be enough evidence that 2015 was no fluke.
Pogba was leaving sooner or later even if we had won the CL. He always was destined to leave and he even went to his "home" club, not to be a mercenary. He would rather be a United legend than a Juve one.
Bayern did keep Ribery but they also, more recently, lost Kroos. We also kept Buffon who had TONS of interest, but lost Pogba. Barca lost both Thiago and Neymar. United kept De Gea, but most importantly lost Cristiano Ronaldo, further back you have Beckham. Who can keep track of the talents that Chelsea has sold over the years. These things happen. The only club where this barely happens is Real Madrid because they've been obscenely rich for the longest time on top of their top prestige.
Alexis has this happening like you say. Another example is Tevez at City. Dortmund has lost way too much money on the table by being stubborn and having their players run out their contracts and leave for free. Schalke is about to lose Goretzka for 0 euros. Its a big risk.
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In any case. We can debate forever, there is no need, we both have our opinions.
A different question. How much is Dybala worth to us? is he worth 80m? 120m? 200m? No amount of money? How much is he really doing for us and how can we use that money to keep improving the team? (4-3-3?).
Unless he majorly steps up, I'd take a massive bid (150+) and be happy.
I wouldn't be upset either if we rejected everything and chose to commit to him.