The Scots overcame Italy in the third-place playoff clash in Pretoria on Saturday, but Johnson has claimed that improvements must be made.
"I'm not walking away from the fact that we have to improve areas of our game, and we can't accept that as a standard," Johnson told reporters at a press conference.
"It has been a tough old tour with injuries and our depth has been tested. That is what we needed to do - test our depth - and this reign is going to continue a little longer because we want to make sure we have a competitive squad.
"It is a hard year, a long year, and you can take pride in the resolve of the boys to finish it off."
Scotland were beaten twice in South Africa.