You do realize that this is only Johnson's 11th year in Cup? He's got a lot of wins and championships ahead of him if Hendrick can keep that team together, which I don't see being a problem.
To put that in perspective, Dale Earnhardt won his 7th championship in his 16th season in 1994 and was very competitive through his death in 2001, 7 years later. He finished runner up to Bobby Labonte in 2000. You could argue that he would've been competitive for at least a few more years.
In my opinion, if Earnhardt could've been competitive into his mid 50s, you could reasonably expect that of Johnson, especially with all the safety improvements that have been made over the last decade. Johnson is only 36 years old.
I think it'll be a while before we start seeing the downward curve of his career.