I would suggest watching videos such as these : Great articles to read on the web - #1012 by sahil_vi to better understand these things. There is commitment from government side for many years of both PLI and subsidies. These are time-dependent and the time duration is announced up-front.
The concern about substitution is true for most companies (probably 95% of them). This does not mean they are not good investments. This is just a risk the investor needs to be aware about and evaluate with regard to the specific company. My 2 cents.