Johnson yes, he could probably put up somewhere between a 4.2 and 4.7 ERA over 170+ innings right now, and that's good enough to be on a lot of teams' rotations.
Yes, he "could" put up numbers like these, and if he did, yes, he would be good enough to make some teams' rotations, but you have to remember, those teams have pitchers similar to Johnson (and Velazquez) who they feel "could" give them a 4.70 ERA over 170 IP.
Johnson has the potential to do even better than 4.20, but so do many SP'ers who won't put up 4.70 or better next year.
BTW, I like Wright as our 6th starter, until he proves otherwise.