It is impossible to know how many people have used bitcoin so far. So all of these researches are just speculations like any other individual speculation. Anyways it does not matter how many people use btc, what matter is the market cap and how mush it is rising
I agree with you that there may be more important questions, but the topic of the thread relates to speculating about number of users, and there are probably some better sources of information than others, but to suggest that the topic does not matter seems to be a bit lacking in perspective.