Tough question. But I'm gonna try and speculate an answer. Let's see, Indy and the LC just wraps up. The studio's are celebrating another success. Harrison Ford goes on to sign a contract based on the success of a "submarine movie" he had previously turned down, and makes Patriot games and Clear and Present Danger.
Spielberg makes his attempt at 'mature films' and Lucas begins work on the Star Wars Prequels.
At this point, let's say two or even three years following LC someone mentions Indy IV. Since LC was still fairly fresh, I'm sure the trio would say "Let's keep it a trilogy". Even if Harry Ford was up to it, S.S. and G.L. would be working on other things 'new' to them and not want to return to familiar territory just yet.
Shoot ahead another 3 years and maybe the perculator starts perking. Another Indy? Maybe... just need a script and a decent story. A memo is sent out htrough the studios about a possible script search for a new Indy film. Spec scripts start piling in. We get Saucermen from mars, Garden of Life, and Law of One.
Meanwhile Lucas commisions his software company to come up with a few games.
Suddenly the fans start to whisper about rumors of a script search for Indy iv. It's not long before rumors turn and say that it's ALREADY IN PRODUCTION! News spreads like hot cakes all over the place. Fan fiction starts to appear all over the web. Unsupported casting claims and plot details surface all over the place.
Eventually it is realized by the filmmakers that the money potential for the Indy franchise is too great to turn down. Indy gets thrown onto a schedule. Now heat is turned up a bit and specific writers are asked to provide scripts... Shyamalan, Columbus, then finally, Darabont.
And now Darabonts done the only COLLABORATED effort between him Lucas and Spielberg. Hopefully the say, "This is it!!"
PS. Moderators, the real history behind the Indy iv developement would be an awesome article for the site. Just FYI.