I partially agree that he doesn't need his theme changed, but I don't understand the rest of this. What you may find interesting is that, among my friends, it's Ziggler, Otunga, Rhodes and Del Rio's theme that is favoured and mentioned by the ones who watch/care about wrestling the least, and immitated and joked upon by us in the group who watch it the most.
I find it amazingly bizarre that you've picked themes that seem to stand out the most and label them as generic.
Personally, I don't find many of the current themes generic. Many of the ones that matter the most just do not seem fresh enough anymore, counteractively, nothing ever "seems" to change now, because of their motions, entrances, results etc... and when it does... it's up for massive critical debate. After like what, 8 years, you're surprised that Cena's theme stands out? I think it's for the wrong reasons.
Basically recognition. Certain themes you don't change. Yeah, HBK has had his theme song since forever but it's a part of who he is. Could you imagine him changing it now? It's completely unnecessary. Like how changing Bret Hart's classic theme to a more modern, watered down version was unnecessary because it kills whatever overness or reaction he would have got if he came out to iconic theme.
To me, I find Rhodes's theme bland or I find Ziggler's theme very dull or I find Otunga's theme unfitting of his character or I find Miz's theme generic. Many of the themes today don't sound the same. They don't give you the "umpht" or " business is about to pick up" vibe that others have. Cena's theme is a drag nowadays, yes, but when you hear it, it makes a reaction (however way it goes, it creates an instant reaction). Punk's theme makes him stand out because it's different and fits his current character. Orton's the same deal, Del Rio's the same deal, etc.
So, yes a change in, say, Cena's theme could be warranted but would it be for the better? Do you fix what is not really broken? He's a face and he's a had that theme ever since Wrestlemania 21. Now, if he turned heel, he would have to change it but right now, that's the least of his problems.