Yeah, I have heard that before (and indeed, the page of the brochure that interests us i.e. page 5, echoes the sentiment), but I find it extremely unlikely that the economy would go into a depression for that long. The governments of the world do have economic tools to prevent that and if the worst comes to worst they can create new ones with legislation - we wouldn't like it, but they could easily do it. Over a long time period, it is likely you will realize a gain. Productivity is always increasing in the world, the only real way for you to lose money over a long period in an index fund is for the world's productivity to start dropping. That'd be bad. Very bad.
I do not share your optimism for the times that lie ahead of us. You base your optimism on a (historically) quite short period of time (100-150 years). There have been macro cycles in the past spanning more than 100 years with economic and population decline.
Given the enormous debt burden of most countries in the world and the increase in military tensions worldwide I would certainly not rely on anything "government". The system is more fragile than most people think.
When it comes to investing nothing is guaranteed. Better stay flexible.
ya.ya.yo!