It doesn't apply just to us
Lion King fans, but all fans of popular media in general. Yes, some actors don't sign long-time contracts to reprise their iconic roles, and they don't have if they don't want to. But, we fans want to see them reprise the roles because we're comfortable with them. You would get the same reaction from people if somebody else replaced Harrison Ford for Han Solo or Indiana Jones in an upcoming movie. Heck, fans pushed for the filmmakers to bring back Michelle Rodriguez for
Fast & Furious 6, and unsuccessfully for Cindy Morgan to return in
Tron: Legacy. And if you read interviews of Moira Kelly about TLK, you can tell she enjoyed voicing Nala and would love to do it again. I'm not sure why Kelly didn't come back since Nala is a minor character in this series and she worked on television before with
One Tree Hill.
As for Rob Lowe voicing Simba, it's just jarring to hear a rougher voice for Simba. Keep in mind, Matthew Broderick was the first actor to sign on to
The Lion King during production, and in an
interview with Don Hahn, he claimed he was chosen because he could play a "kind of character who could be irresponsible and likeable, but you also felt that he could come back in a very heroic way", and while it doesn't please everyone, it's understandable because adult Simba starts out immature and his boyish voice proves it. I understand Broderick has a busy career, but I think there's no excuse why Cam Clarke couldn't do it.