Nah, better answer: the demo isn't a priority for Firaxis, compared to the game's release.
True.
Preloading is a bit silly though, since it makes the release date pretty irrelevant
Preloading stops everyone trying to download the game at the same time. The release date is still relevant because (i) that's when retail versions are sold, (ii) that's when the day-zero patch will be ready.
(the game is obviously ready to play now).
You don't know that. Now that the game has gone gold, Firaxis and the beta testers will be checking for any missed bugs. Even
if the game was ready to play now, what do you expect them to do? Retail stores probably haven't even received the boxed versions of the game yet. Do you think they can just tell Steam to unlock the game now?
If they need a day-zero patch for serious bugs, we're in for an uncomfortable release-date gaming experience
Why do you assume that? Equally, you could argue that since a day-zero patch will fix any missed bugs, the release-date gaming experience will be excellent. That's one of the benefits of Steam: if any last-minute bugs are found, they can be patched before you start playing the game. I'm not for a second claiming that civ5 will be perfect at release - I'm sure there will be several patches over the coming weeks and months (not least because 100,000s of gamers will inevitably find things that beta testers missed).
I remember when I bought civ4, it kept crashing. Eventually I found out that there was a patch available to fix the problem (yeah, I was a bit slow). But I'm sure that a large percentage of people who bought civ4 did not realise that a patch was available (if they even knew what a patch was at all) and just got fed up playing the game. Now with auto-patching in Steam, those problems will no longer be an issue.
Only a week away now, but it's still a bit annoying they couldn't release a demo before the release date, like they said they would for weeks.
You're right, it is annoying - it would have been nice to have a demo now. But, like you said above, getting the game released on time is the main priority. Clearly they decided it was best to wait for the game to go gold and then make the demo.