It's not entirely sunk cost - there's reputational effects for a lot of people involved. Politicians who supported it look much worse if all this work (and money) has gone into it, only to have it not launch. How bad they will look is proportional to how much money was spent/work was done, so the work put in does matter to the outcome, at least for them.

That probably shouldn't be the case, but when it comes to politics, perception is very much reality in that people's perceptions translate directly to votes.