I didn't suggest that Admins should pay for anything, let alone everything. And unless your contributor agreement is substantively different than mine (which is certainly possible), I know how much we get paid for writing AL adventures. I'm sorry if suggesting that Hasbro pay you to attend a con is somehow offensive; that's not the intent. What I'm saying is that we have an existing channel in which content creators are paid to produce adventures for the AL community as a whole, and I find that preferable to devoting resources towards producing adventures for a very small subset of AL players.
To your own point, if con attendance costs so much that writing an AL adventure doesn't cover it (which is certainly true for some cons, depending on size and distance), we shouldn't be passing that financial burden on to players who want to experience AL content.
I understand the question, and Day 1 is the period I think is right; I don't see any value to the broader AL player base in enforcing content-exclusivity behind a convention paywall.
Similarly, if we were to suggest "This adventure can only be run in Japan until January 1, 2018" or "This adventure can only be run at stores that have sold 50 copies of the SCAG" or "This adventure can only be run by DMs who are certified Level 3 by the Herald's Guild", I would say that this exclusivity feels artificial and intended for the benefit of someone other than AL players. And that's not awesome.