If Bitcoin is used to pay salaries, is it necessary to collect taxes?
It depends on the government of the country. In a few countries, there are taxes for crypto coins. But in my country, there is still no regulation of crypto/Bitcoin taxes. When Bitcoin is already functioned both for investment and payment tools there, the government probably plans to apply taxes.
If it is more convenient for everyone to buy bitcoins and use bitcoins for daily payments, will the price of bitcoins get lower and lower?
The value of Bitcoin cannot be compared with fiats. It is more similar to Gold. You must know the gold value, right? Even if it is used as a payment tool, the value of Gold is always high (expensive). It also probably happens to Bitcoin, the value of Bitcoin can be still expensive when it is already used as payment tools daily.