Seems to me that the scene was intended to be reminiscent of Elsa's demise in LC. Another character (though better written) with dual allegiances. Elsa's choice was more obvious....erm, obviously. With this in mind, I prefer think this was a conscious choice on Mac's part. After quickly realizing that his goose was cooked and that he would only take Indy with him if he continued to hold on. Finally a chance at redemption with his old friend. With the temple collapsing, he could no longer ally himself with Spalko, anyway, making his choice in the matter that much simpler. Much like Elsa siding with Indy at the moment of truth, for totally different reasons, I believe Mac's character was also intended to come around to Indy. The Mac/Elsa greed in their respective films' climaxes (Mac stuffing his pockets/Elsa attempting to cross the great seal) are another moment that these character's arcs seemingly mirror each other.
So I'm going with intentional here, regardless of how murky or ham-fisted the scene was presented in the final film. Since I haven't read The Complete Making of, or the novelization, perhaps some insight could be gained by a Ravener with these sources.
For now I leave you with this excerpt from the
final shooting script, that seems to give credence to my theory of the film's intent:
INDY (cont’d) *
Come on Mac, use your feet (legs). I *
can’t do it alone. *
Mac’s face falls, there’s no way out of this. *
MAC *
Jonesy I’m gonna be alright.
He winks. He lets go of the whip, his body is immediately sucked, *
into the void
Of course, none of this is to say that the original intention of this scene wasn't altered on the fly, or in editing.