Again, we've all seen beloved characters be disasterously and abruptly changed into completely characters so they can be used as a villain. It goes terribly every time. So, given that this idea goes so strongly against Turalyon's past and current characterization, strong opposition is to be expected- nobody likes those extreme character derailments.
And yes, I've seen it get brought up for years, but that doesn't make it fitting. There's always people who aren't familiar with characters and make suggestions that don't make any sense. I mean, there were always people who thought Sylvanas cared about the Forsaken despite her inner monologue explicitly spelling out that she only saw them as weapons, and then later as meat shields to protect her, once Arthas was gone and she no longer needed revenge. Those misunderstandings made it no less jarring when she suddenly did care in BFA. And then didn't care again at the end of BFA. And then cared again at the end of Shadowlands. That's the kind of inconsistent characterization you're asking for. Of course people are strongly against it happening again, after seeing past disasters like this and Kael'thas.