Reply to "'It' vs 'this'"

It, this and that in paragraphs - English Grammar Today - Cambridge Dictionary this site says that both it and this can be used to refer back whole sentences and clauses. This highlights the information referred to much more strongly than it.

So here is a sentence from the above site, which is written using this. Can I also use It there?

More and more people are discovering that Tai Chi is one of the most valuable forms of exercise. This/it has led to a big demand for classes.


If it is right then why can't I use it in my sentence 'The government has decided to plant one million trees in the city. It/this will improve the air quality in the environment.'?
 
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