I think many people do that - it's very hard to write up a compelling proposal without having spent some time working on the idea to see if it's viable, but there's often no funded way to do preliminary work on an idea. If you don't do something like what you describe, it's nearly impossible to branch into new areas, and that makes it nearly impossible to do good research. This is another problem with funding project proposals.

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.