Why shouldn't it just be three films?

Well, again, it goes back to my thinking about the James Bond movies--what if they only made 12 Bond movies based on the 12 novels? Then, we wouldn't have some of the other considerable Bond films not based on the novels (for better or for worse). Or what if they just stopped at Sean Connery?

Sure, we've gotten some stinkers when it came to the Bond franchise but we also have gotten some considerable achievements from those not adapted from the novels.

I'll just leave it at "I think Indy is to have additional adventures just like Bond is to have additional missions."

But I am with you on the concern of people taking the character and having it "peter out into mediocrity."