In R&D, incentivizing a private enterprise to spend more could be beneficial. Assuming the client (NASA) has some oversight this leads to more engineering freedom and an exploration of a larger design space, whereas fixed cost would incentivize racing towards the best solution as fast as possible.

Depending on program goals, both approaches have their place. However for projects like Artemis which tried to minimize R&D cost and save time through component re-use, the latter approach would clearly be better.