To those complaing/worrying about turn out, I think you're misunderstanding why it's so large at the moment; it's because the way the events have been advertised, the way the content has been set up for only the most hardcore previously, and frankly the poor state of Elite for so long, a lot of people will be turning up assuming this is the swansong of the Thargoids, possibly even the game, and this is the only chance they'll get to do any of the content before it, and possibly even the entire game is gone.
That it's not as good as "Insert Content Too Complex For Casual Players To Have Seen Here" isn't really an issue that healthy games should be concerned with; believe me, I've worked in the industry (EM content writer for Ultima Online) and you'll always get huge surges in players for one shot events; what the issue is, is that Thargoid content just hasn't been engaging for years to players who'd want to see it otherwise. There's been no way to play it that is persistent and low key. If some of you get what you want, and that last Thargoid mothership is permanently destroyed and the content is gone again, yes player engagement will drop off... from the game as a whole.
How would I tackle post-Sol victory (it will be victory, you can never write for anything less than temporary set back)? Thargoids retreat. in doing so a new mystery is found (tech left behind in the panicked fleeing from Sol) which will show their homeworld, content is moved to there permanently and those of you who love it as is get to fight to constantly keep them contained in a bubble away from human systems...
... And I'd add in a Ship Module that can now be unlocked that, outside of Thargoid controlled systems (where their corrosive fields leave them nice and buff, but away from that it's a lot of underpowered probing) an average player can take on every form of them, or at least go sightseeing. And give it a mechanic that opens up the map and highlights where it can be used in really, really obvious ways. Press the honker, get long distance pointer. Make it simple to find and engage with at first, make it permanent, leave the hardcore in game for those who want to always be an AXI player but there's a much clearer ramp up to what to actually do.
Whether you'll get that depends on the competence of Frontier as a whole. Not a lot of players outside the forums believe in that, which is why we're all piling in now, when it's easy to sit outside an obvious zone and see most things.