It's a model, and a lot can change. In that time frame, it's actually quite reasonable to expect we'll develop effective vaccines and treatments. We might be able to roll out comprehensive and fine-grained testing (as has been done to some extent in countries like South Korea and Singapore).

The alternative is to just let it burn through the population, hoping it takes out mostly economically unproductive people (the sick and elderly), and letting business as usual go on. Some might find this an acceptable policy, but I do not.