But what I was describing was slightly different - once the proposal is funded, many PIs feel compelled to deliver what they said they would, whereas in the two years since you wrote the proposal, the world has moved on, you've learned things, both from your early work on the project and from others, and more promising avenues are now possible. If you feel compelled by the funder to continue in the original direction, you'll very rarely deliver good research.
Many of the most critical research results came out of some random hallway conversation, as a response to some new result that didn't exist when the research plan was formulated, or because someone had a brain fart. If at each point someone had put up their hand and said "sorry, scientist, that's not on your current grant roadmap" we'd probably still be dying of preventable diseases and lighting our homes with gas fixtures.