^ Why not choose BTC alone?
All you have said above was the usability of BTC and for sure there is more usability that can BTC do but usually, people choose to store value than use it as a currency. In altcoins, they have different functions, you can use them all in one coin only even in ETH.
Sorry but I did not hear any when it comes to payment, only BTC can do and is widely accepted.
Well you can but as the price fluctuates and you buy with your gains, every time you make a purchace you need to reserve taxes to pay for that too. Unless you are trying not to get caught by irs or who ever deals with taxes on country you live in.