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.
And in 30 years, no project officer from a funding agency has ever complained that we'd delivered really good results but they weren't the results we'd promised. They're generally happy something good came out of the funding, because frankly, most funded projects don't produce much of value. Very occasionally (mostly on EU projects) we've had to write up the results in a project report with the original section titles that no longer match the new content to tick some boxes and keep the funder beancounters happy.