Well, this would only be possible if the 100 people worked under different IP addresses and user names (if they are logged in). The number of edits by a single user/ip is restricted to, as far as I remember, 6 or 8. But however, you're right, such attacks could happen and they in fact happen. But on the English language Wikipedia, for instance, there are a few bots (little computer programs running on Wikipedia) that automatically revert obvious vandalism. If a user removes all content from a page, this will be reverted by an anti-vandal bot after just a few seconds. The same happens if you add well-known insults or nonsense ... (you can't imagine how many people think it's funny to add "penis" to articles ...). Besides of that, on en.wikipedia there are more than 1200 administrators (users with the ability to block users, delete pages and protect pages against editing). You can be sure that someone will notice such attacks.
Indeed, changes are displayed immediately after they're made. There are, however, many users (not only administrators) who go through the recent changes and "review" them. For less obvious mistakes, users have personal article watchlists. For example, a user who is skilled in English history puts articles on his watchlist that refer to this topic. When he checks his watchlist, he will notice that someone has changed the article and he can review the change by one click.
You see, vandalising Wikipedia is not that easy as many immagine.