For me, I'll say that as bitcoin is the first  of its kind, it is useling its dominant power to subdue others, and bitcoin itself has lots of backers that bitcoincash, bitcoin gold, bitcoin segwit2x does not have. Bitcoin's influence in the crypto market is so high and trusted that it affects all other currencies.

I think you cannot speculate the number of people using bitcoin exactly. Because, a user can create many accounts as his wish. If someone can say the total number of bitcoin user, it might be the total registered users (of an exchange or an online wallet) of a company, not the actual number of peoples.

the value of thing depends on how the value for everything because people agree so,the same is true for fiat...in reality fiat is only a piece of paper and we all agree it has value,the only difference between fiat and btc is who backs what. What scares people the most is that btc has no issuing authority...they feel unprotected using it but so far btc has proven itself to be formidable