If Gus was seen as a NHL starter, and the Flyers weren't willing to give him a chance, other GMs would have offered middle round picks for him, the way Hextall moved McGinn for a 3rd, Marody for a 3rd, and Cousins and Weal were traded for late round picks. These kinds of trades are made all the time.
Not many trades for guys playing in Euro leagues with no transfer agreement. Kubalik is about the only one and a top 6 talent... not a guy with a #4-5 upside. They would have had to offer and then wait a year to bring him over... as he was never not under contract for a team without a transfer agreement.
He waited until right at the deadline in 2014 and the Flyers qualified him so he could not be a UFA.
Then in 2015 was in talks with the Flyers but they thought they had too much depth, and then in 2016 just gave up with them... and then they came and said they wanted him after he had signed a KHL deal already.
Gus did not help himself by basically just getting fed up with the Flyers and Berube over 2014... but he should have been a regular from mid-2012 and it must have just worn thin. I think everyone knows he played himself into the #4-5 role and was just arbitrarily taken off the team for weeks and months at a time when had played well. Then #6s brought in to replace him constantly when he had done nothing wrong.
He expected them to not qualify him I believe initially, and then when they did even though they did not intend to sign him he signed a one year contract so could come back next year if given a spot on the roster.
And Euros turn down NHL deals more than a lot of people in North America think and it just does not get publicised. John Norman and Pius Suter in very recent history from what I know.