The lifespan issue is the main reason I hold out for the possibility that Shaxx could have created the Crucible, but do you really think it's likely?

There are SO many thing that have to happen precisely in order for that to happen.

In the Last Word 3, the way it's written suggests that Shin is still pretty young. So, even if it's 20 years (maybe longer as you say) or so after Loken is killed, we're talking about the wall being erected in the mean time and the Fallen attacking soon after the wall is erected. That's some pretty stupid tactical planning.

There is no cross-reference between the first group of Guardians (Kabr, Pahanin, Yor, etc) in the Grimoire and the next (Eris, Eriana, Cayde, Toland, etc). Again, it doesn't definitively rule out the possibility that they knew each other, but really? No references at all?

It diminishes the sense of ancient lore the game gives us. We're interacting with people who helped construct the wall? Then these events aren't lost in time and the major players' reminiscences aren't buried in ancient archives. All we have to do is mosey on over to Zavala and ask. That doesn't seem right. What's the use of having 1000 years of lore if the only important things happened a total of 15 combined years? Seems cheap.

I had not heard that the Golden Age was only 40 years. I'll have to watch the ride along. I believe you, but I also don't see how that's possible. How do they know that human lifespan tripled "during the golden age" if it didn't last long enough to find out? They turned Venus from a poisonous sulfur-stormed lava planet into livable conditions AND built cities, research archives and colleges within 40 years? Considering it took Titans hundreds of years to build the wall (even considering diminished light and resources), I don't see this happening. In that same time, humans also terraformed and populated Mars and Jupiter's moons. It's just too much to swallow and if Bungie's answer is "space magic," they need to get a little better at storytelling.