Even if the Avengers weren't involved, human ego would have made the Phoenix 5 go too far with their belief that these were free wishes, and even if they were able to keep the cosmic being and not get destroyed, the need for energy and where to get it couldn't be avoided and the outside universe would have to be continually sacrificed to maintain their perfect world, because the Phoenix needs sustenance according to the Kree's Supreme Intelligence, Thor's hammer attack seemed to have proven this. There were clues to show that the Phoenix wasn't some genie out to make one world of the whole universe a perfect place. The X-Men were arrogant to think that they were more special then the rest of the universe and the Shi'ar knew what a major mistake they were making and Hope knew it when she had a taste of it's addictive power.
Something was bound to go wrong if the X-Men kept the Phoenix, which they realized later on, and Hope, not Cyclops, made the right choice of letting it go which restored the mutant race which Cyclops thinks he deserves some credit for.
The Avenger may not have had all the information when it came to the Phoenix, although they had Beast with them, but Cyclops was more wrong then they were in the end and I hope Hope confronts him to tell him that it was him who almost blew it. They'll probably will since I won't be surprised if he wants her to be the mutant messiah again and lead the new mutants like they were sheep, since he seems to be losing it and drinking the kool-aid like Wolverine has been saying.