The problem with this is the potential for massive gaming. The argument of non-startups would be why should they be penalized for having hired people before this period or are considering laying them off and would not otherwise be viable without the tax advantage?

Ideally they would make the tax incentives permanent - ie reduce payroll taxes if the goal is to increase employment, but I think the startup visa is a much more interesting alternative as it seeks to attract talent that traditionally is difficult to define but is so important to how vibrant an economy is. In theory, just the fact these people bring viable ideas that people want to fund should be sufficient irrespective of jobs they create.