Would some choose not to create works because they wouldn't get that deal in a hypothetical world where we reigned-in copyright terms to pre-1976 durations and life-plus clause? Yeah-- maybe. I don't think the effect would be terribly chilling. There was a lot of work made available under the 1909-1976 regime.
I certainly don't think we got an explosion of new work because of the 1976 act. It was a windfall for existing rightholders and a method to limit future competition.