Don't worry about that. Check a look at some of sammy2's posts, if you're not familiar with.
Anyway, this is clearly hugely premature. Johnson has been an oustanding Test bowler and more-than-useful lower-order batsman... for the last 8 Tests. Whether aussie's white, or blue, or red, grandad thinks he's got the talent to match Davidson is really neither here nor there, as all sorts of people think all sorts of players have the talent to match other players.
Maybe he will, maybe he won't. But until he's actually covered a fair amount of the airmiles of his Test career, there's really not much point talking as if he had. He's barely off the runway. "Can Mitchell Johnson be as good as Alan Davidson?" would've been a far, far better thread title.
One thing to note that they both have in common is a relatively advanced age. Johnson was 27 before he became Test-class; Davidson was 29. Davidson also played on for just 4 years after that, retiring at 33 (that wasn't unusual for Australian players in his day). That's the reason he's never considered up with the very, very best of the seam-bowlers.