To all you jailhouse lawyers out there who are trying to quote contract law to make excuses for Captain Sim, how about we just focus on simple ethical behavior.
I own the boxed version of 757 Captain from Just Flight.  
Captain Sim’s name, copyright, “trademark Aircraft Configuration Editor” brag, and verbiage is all over that product.
You can assume that Captain Sim profited by either getting a piece of every unit sold by Just Flight, or else getting a pretty hefty licensing fee in the original deal that allowed Just Flight to box and sell the product.
For them to walk away from supporting the product just because they apparently couldn’t reach terms on an extension of that agreement in no way should absolve them from continuing to support the product that has their name on it.
And it does need support.  The 757 (my last and final purchase of a Captain Sim product ever) is, like most Captain Sim products, annoyingly flawed.  One only has to read the forums on any CS product (most notably the 727) to see that purchasers are constantly forced to either accept bad behavior by their purchases, or devise workarounds that should have been fixed by the manufacturer, the efforts of CoolP and Markoz notwithstanding.  
I own the 727, the C-130, the 707 and the 757.  Each of them requires more time and effort to fix, update, kludge and repair than to actually fly, which is why I say the 757 is my last CS purchase.  The support just isn’t there… and I'm done buying the hype that THE NEXT plane is really, really ready for the public.
CS has a great publicity machine that makes every new aircraft sound like the Second Coming.  Delivering on those promises is where they fall down.
But the real point here is that hundreds (thousands?) of us have purchased this product on the Captain Sim name, and for them to walk away under a pretense speaks volumes about their corporate ethos, and what you can expect in the future if you continue to deal with them.