Responses
I think it because it wouldn't end their suffering. Even if they say "Oh we no longer have to titan abilities therefore we are not a threat" some people may not believe them. Another thing is that even if they do believe them, the eldians are being persecuted because of what their ancestors did, in other words even if the eldians didn't turn into titans any more their suffering my not end. It will take away the only advantage they have.
If the founder titan can manipulate eldians bodies to resist diseases , and zeke wanted to use that power to make them unable to bear children thus ending their race , why not use that power to make eldians not turn into titans and became normal like other humans , rather then killing or making them go excitended