I would flip the question on its head and ask why the response to uncertainty is to default to an assumption of zero. Environmental benefits are notoriously hard to calculate and it's unfair to weaponize that difficulty as if it confirms the default assumption should be zero. I also don't think the calculation needs to hinge on one-for-one timed tradeoffs in the form of "this windmill exists to replace power from that dam" or assume that backfill is 100% fossil fueled, given that solar is now arguably the most rapidly deployed form of new generation.

It's not that the cost never matters but I think if the cost is really just $25MM for a transformatively beneficial environmental impact that has equivalent no alternative, in exchange for scaling back a fungible resource, is a no-brainer.