It was just a matter of who was going to whack Ralph, he was living on borrowed time. I'm surprised he lasted as long as he did.

Gigi probably would have plotted his demise if he didn't die.
Paulie would have eventually obtained proof that Ralphie pranked his mother.
Johnny Sack might have taken matters in his own hands after Carmine Sr passed away.
He might have been a closet bi-sexual based on his sexual tendencies.
He was disrespectful to Tony. I believe the horse was the straw that broke the camel's back, after all of the problems that he caused.

Vito took over the construction site and became the top earner. So, it wasn't as if Ralphie couldn't have been replaced.