It's the opposite.
I was pledging one carrot. But to get you that one carrot today, I have to pay as much as for 10000 carrots back then.
Next time I will say I pledge whatever X USD will be in Bitcoins when it is finished ...
That's besides the point.
You promised a carrot, not $400 worth of carrots. And the developers were committing their resources expecting to get the full carrot. How
you get hold of that carrot is your problem.
But perhaps this could be seen as exceptional circumstances … I don't want to pick a fight here. Of course it’s totally up to you how much you give; it’s your reputation you are risking in the end.
This should be a lesson to all of us to ensure the credibility of future bounties.
Either reserve the pledged BTC in a place where you will never touch them (like I have done).
Or denominate your pledge in a different currency.