If they booked him like they did in 1998 consistently how long do you think it would have been before nobody cared about him any longer? They had to change it up to keep him relevant IMO.
My response to that is to look at the Undertaker. When he debuted, people just thought it was another cartoony gimmick and Foley once noted he thought it would be a 3-4 year character at best. But he gimmick has lasted throughout the years and he's been one of the longest running gimmicks the WWE has ever had. He did switch to the American Bad Ass but only for a few years. If they can do it with Taker, don't see why they couldn't have managed with Kane. He wouldn't have looked like a carbon copy either, Kane is different enough to set him apart.
I liked it when they had Kane finally speak regularly and showed that he had a human side with the way he responded (the night after Fully Loaded 1999) to Taker destroying X-Pac or his love for Tori or even the way he was distraught over Snitsky causing Lita to have a miscarriage. His unmasking was also awesome (burning JR, frying Shane's nuts, no selling Austin's stunner and chair shot and laying him out, burying Undertaker alive, etc.) His friendship with RVD was similar to the Bryan/Kane relationship, except not nearly as comedic or corny. It was also part
Him slapping Bryan on the ass or doing a rap video with Bryan or a number of other things, I do not care for.