Agreed. If the government was serious about starting a program for reducing emissions from internal combustion engines, it could start by gradually increasing the gasoline tax until it became prohibitively expensive to buy an internal combustion engine car in 2030. In your analogy, the diet would start on day 1 with an objective reduction in sweets, even if not 100% quite yet.

BTW, I'm not saying that the government should increase the tax, I'm just saying that a concrete action in the present would go a long way towards building credibility.