I am not a Mordekaiser main, but I am certainly opposed to the title change, even without considering their recent modifications to Morde's gameplay. There are plenty of people, including myself, who develop strong connections to the lore of Runeterra and choose champions largely based on their lore and thematics. They may not be a majority of the player base, but they are passionate because of their investment in the world. Think comic book nerds or the kids you knew in school who always had a novel to read in their free moments.
I'm sure you would understand if people were upset by Harry Potter's scar being made into a skull instead of a lightning bolt or Peter Jackson deciding that hobbits should actually be tall, since those changes would strongly impact the nature of the stories and characters of the Harry Potter and Lord of the Rings universes. Changing champion lore, including titles, art, VOs, and things like that, have the same impact on lore fans because of our investment in the characters of Runeterra. Even seemingly small things like title changes can indicate that a character has undergone a fundamental change and will likely move more in that direction in the future. This is why title changes can be so concerning, especially in a character who is likely to receive major lore updates in the near future.