People have become better at capturing value for themselves.
> Why do there seem to be more examples of rapidly-completed major projects in the past than the present?
Culture, competition and selection bias. There are still a lot of successful projects being built, just less so in culture that don't do cooperation well at this point in time. See previous answer.
> How do you ensure an adequate replacement rate in systems that have no natural way to die?
Reform(s).
> How do we help more experimental cities get started?
It has become common to cite those cities as successes without actually having spent much time there and especially not on a "grassroots" level. Chances are that it is actually both better and easier to reform existing cities or areas.
> How do people decide to make major life changes?
Like most things people do things when they are easy. Best way to make a major life decision is to make it easy to do so. Very few people are e.g. moving to countries they know nothing about.
> Why are there so many successful startups in Stockholm?
Stockholm is less worse than many other places, especially so in the end of the 90s, early 00s. Most places in the world are really quite petty and idiotic.
> What's the successor to the book? And how could books be improved?
Multimedia like Microsoft Encarta and An Inconvenient Truth. (There is more to it of course, but that is the direction).
> Could there be more good blogs?
No, journalism has a low enough barrier to entry now that blogs as such are largely obsolete.
> Why are programming environments still so primitive?
Because the stakes are low and there is enough low hanging fruit elsewhere.