From what I understand, the journey of the Gnosis was a player initiated decision, to bypass a region of space frontier boarded off, yes? And frontier responded by locking down the destination area, but the player group insisted on the destination?
Does it not seem that perhaps the developers locked the region out with a plan for future content, and just haven't finished the stuff they were planning on putting there? I mean, development doesn't happen on the player's timetable. Expecting a player-initiated event to encounter new content on the scale of an entire new region seems a little off base. Letting them try, fail, and get to face off against new thargoids for their trouble is already pretty impressive for player driven narrative, compared to the alternative of just saying "no, you don't get to try, the thargoid station-motherships aren't ready yet".
From what I understand, you're almost entirely wrong. Cannon proposed a jump to an unlocked sector behind a permit-locked wall. They put that to FD; FD said, sure, go ahead. Then FD locked the unlocked sector but said the mission could go ahead. None of this is on Cannon.
FD had almost unlimited options at that point - the obvious Thargoid intervention (which about half of the users expected); some mysterious reveal (which many of us hoped for); a simple nothing (which the other half expected); some unexpected 'other' (which almost none of us expected but a great many hoped for). What we got was the least interesting, least creative option - and then they blew it hours ahead of the actual event through sheer administrative incompetence.
At any point, FD could have said 'no' to Cannon. What they have engineered is the worst - the absolute worst - of all possible outcomes.