Except that if they don't follow Indy, they don't get the headpiece in the first place.
I admitted that in an earlier post:
Second, Toht only got hold of the medallion in order to unearth the Ark because he followed Indy to Marion's in Nepal.
You are correct but once that happens his involvement in the plot is done.
Plus you are overlooking the overall theme: preventing the Nazi's from getting the ark in the first place. On that, he was successful.
That is the point tho, by that logic if my goal was to prevent the Yankees from winning the World Series and they lose in the first round of the playoffs would you say I was successful? No, of course you would not.
The key is that at the end the Nazi's had the Ark, Indy failed to prevent them from having it and the only reason it did not end up in Berlin was because Belloq insisted on opening it and when he did they all died.
I still love the movie, listened to it at work today in fact, but it was an eye opener when that was brought up in the show. I thought to myself, "Yea, she has a point there."
"The key is that at the end the Nazi's had the Ark, Indy failed to prevent them from having it".
But he did not. Just by being there and survive, Indy assured the fate of the ark. Without him being at the site, the only thing that could've happened is that a different nazi battalion would've gotten ahold of the ark in the following weeks.