"he is gradually becoming a disappointment"
"Age is beginning to show ..."
I can understand the disappointment. On the other hand, I don't think anyone has a right to think Johnson has let anyone down unfairly. The organization knew how old it was when they hired him -- they placed a premium on reputation and buying the current best, rather than the future best. It's exactly that kind of 1-2 year rotating door mentality that has served most "rich teams" that employ it poorly.
Like any pitcher on the downside of his career, Johnson is going to have his streaks where he looks "like the old Randy", mixed in with the streaks of "holy cow, what happened to this guy, he looks like he aged a decade overnight".
For all Johnson's blustery exterior, I think his confidence and psyche are fragile. When he's "lost," it builds to a downward spiral as his confidence is clipped and that in turn affects performance negatively.
On the flip side ... he's never more than a couple back-to-back strong outings away from one of the "good" streaks.
The key is to fix the mechanical flaw that's causing him to rotate the hips open ahead of the arm delivery. When that's fixed, he'll be far less concerned about pitching in to lefties, and about letting things fly at full followthrough and velocity to righties.
p.s. your feeling about middle relief ... I'm not sure there's a team in baseball whose fans feel much differently. Generally speaking, the best arms go to the starting rotation, the ones with less ability to "pitch", or with one strong pitch but no balanced arsenal, are sent to the ranks of closers, and the lowest quality guys get consigned to "middle relief". If the worst pitchers on any and every team wind up falling into "middle relief," it's no surprise they cause angst among the fans (and managers!).