They will tax it just like they tax it now.
People will still need to work and get payed and companies will still have to write those payments down.
If you own a shop right now you can also sell under the counter, nobody is stopping you for trying.
Just like you can get caught now you will get caught using bitcoin payments.
if you think that with bitcoin everybody will avoid taxes you're deluding yourself.
Oh , and until we solve the 300k tx max a day , we won't have an increase in bitcoin payments.