I would simply comment that slowing down the process would just shift the impacts into the future and spread them out over time. If that same development takes place over 30 years, then obviously all of the annual impacts would be spread further.
The oil will be in the ground until it's exploited. Depending on the rate at which it's exploited, we're able to estimate what it would do to the economy. With no judgment on what rate it should be exploited at, we take what we think really is happening and translate it into economic impacts.