Hopefully both Sony and Bungie are looking at Marathon as a project that will have 10+ years of updates. And Bungie isn't only Marathon. About 1/3 of the studio is working on Marathon, while the rest are working on other projects.
And Sony I'm sure knew it would take 10+ years before they saw a return on the investment of Bungie. The 3.7bil shouldn't be a concern as the money is already spent. Sony isn't going to start laying off Bungie employees if Marathon is only a minor success. It doesn't need to be the next massive hit for Sony or Bungie, especially at launch. Marathon needs to show potential for long term growth. That is probably the expectation for Sony and Bungie at launch, or at least it should be