The most telling thing in Marshall's account is that they clearly have a timetable in mind. Unlike the long delay to Indy 4, everyone is eager to make a sequel
quickly. The only time this really happened before is when Ford issued a "now or never" ultimatum back in 2006. And of course, a few months later, the movie was officially announced.
This also helps to explain why both Spielberg and Ford have been reluctant to fill their schedules. Each man usually has several projects in the works- even if it's something they're merely rumored to be circling.
Ford, especially, has been surprisingly slow to commit to anything that requires heavy lifting. He desperately needed the career boost that Indy 4 provided, yet has been very slow to capitalize on its success. It certainly looks as though he's keeping himself available for another installment.
Johnny Nys said:
In CS, Spalko reveals their intentions during the hypno-scene. Nothing immediately world threatening about it all.
I think it just depends on how seriously one views the Cold War. Admittedly, a lot of younger viewers probably couldn't relate to the idea.
But Spalko's explanation is certainly more substantial than anything Mola Ram offered up, and is even backed up by a world superpower. Are we really going to pretend that "toppling the gods" is a better plan than Red Scare intimidation?