The variables aren't exactly constant. Our models, such as the general equilibrium model, take economic changes into account.
The costs are calculated between now and 2030, a relatively short time frame in an economy such as Canada. We often say in briefings that nothing shows that there won't be any economic benefits after 2030. However, between now and 2030, the costs are measurable. The benefits of a transition to a less carbon‑intensive economy won't be felt yet. It's relatively brief. There isn't time to meaningfully introduce new technology that can offset the negative impact of a transition to a less carbon‑intensive economy.
For example, many assets are already based on a fossil fuel economy. If we eliminate them quickly, there will be a cost. We call these sunk assets. Eliminating and retiring assets before the end of their useful life carries a cost. This explains much of the costs.