Sunk costs will be a consideration, but so too will the fact that the Kilo's average procurement unit cost is over 4X that of the F-model Hook ($117M projected vs. $27M as of December 2013, per the latest available Selected Acquisition Reports).
The existing CH-53G infrastructure may be of limited relevance to the Kilo, given that it's a completely new aircraft under the skin (which, being composite rather than aluminum, is also new...).