I don't think it's a sunk cost fallacy. I think the hesitation is more for social reasons, often not wanting to do a big pull request that's going to be scrutinized.
"Big pull requests" that are unannounced are always problematic because who wants to be the person saying "all of this work you've done is wrong"?
In such situations, it's good to get buy-in from other people before attempting to do such a thing. Make a proposal for a big change and discuss it. There's still a chance that, in the implementation it doesn't work as nicely as believed initially, but at least now it's less likely that the idea will be rejected wholesale during code review.
