Nope, not sick of it yet - hence why I want it back.
I get what you're saying, but at the same time, I don't think it's so cut and dry as you make it. First, some parts of a character's identity shouldn't change much if at all due to their importance. You don't see anyone ask, "Does Peter STILL believe that with great power comes great responsibility?! Jeez! Get over it already!" The concept is integral to the character; similarly, the idea of clone degeneration is what made Kaine special and different from his "brothers". Now, he's just presented as "Grumpier Ben Reilly".
Furthermore, being "stuck" with his original status won't stop Kaine from growing and changing as a character. Characters, like real people, grow and change in some ways and stay the same in other ways. Not ever becoming a woman hasn't stopped me from aging. Kaine never escaping from clone degeneration doesn't mean he can't learn to be a better human being - and in fact, if you are invested in his growth and redemption, wouldn't you rather see him trying to be better while still struggling with his greatest fear? Easy to try to reform when you're no longer a scarred monster.
So, while I totally agree that stagnation is certainly something to be avoided, there are still certain qualities that become ingrained as vital to a character's identity that shouldn't be ignored and done away with.
-Pav, who believes in happy mediums...