I personally think it is overstated that most of our content comes
from short-term contributors. What I do find is that short-term
contributors begin a lot of our articles, yes, but they generally
produce content that is not up to a high standard. My experience is
that in most fields of interest, large numbers of articles are
substantially written by the same small subset of users.
On the other hand, these people have to start somewhere. I think that
people get into Wikipedia in one of two ways. The first is that they
hear about Wikipedia as a project and go there because it sounds
interesting. The second is when they find an article that's not very
good, and improve or replace it.
In the first case, people are likely to read featured articles and
other stuff pointed-to off the main page first. In the second case,
they are less likely to - although they are likely to poke around
similar content till they find a 'good example' to work from. In the
first case, the FA process is very valuable - we're showing people
examples of what to do to create a good Wikipedia article. In the
second case, applicable WikiProjects are good, because they improve
the chances of someone finding a similar article to a bad one that's
done properly.
Articles don't develop linearly - I agree. However, what generally
happens is that drive-by users add snippets over time until someone
wanders by, thinks "What a mess!" and decides to yank it out by the
roots and start over. However, that 'start over' will generally
contain most of what's worthwhile about the previous version - people
are averse to dropping content (even when it should be dropped). As
the article gets longer, these rewrites tend to be of sections rather
than the whole article.
-Matt