Well, they chose to announce the title on MTV, and George has talked to MTV a few times. They seem to be targeting a younger audience.
But that only makes sense. I'm in my late 20s, just past the typical MTV crowd, and I grew up with the films (even though I only could see LC in the theatre). So for everyone my age and older, Indy is already ingrained in our social consciousness as a classic character. We don't need an aggressive marketing campaign, just being made aware that the film exists will be enough for most.
The younger people might need more than that...a lot of them might be familiar with the films, but there are probably many for whom Indy is a vague character from "those 80s movies". They need not only awareness, but something to draw them in to what is essentially a new endeavor for them.
I'd like to think the idea of introducing Indy's son was independent from marketing ideas, but once that was on the table, it would only be smart to choose an up-and-coming young actor who is recognizable to the MTV crowd. There will always be haters but I think Shia does have his own special talent going on, to boot. And honestly, I can't think of any other young actors who would really fit the bill (except maybe Emile Hirsch).
If it gets younger people watching the original trilogy, who can complain about that?