Going back to the Hobbit example I would not support...
is not an example - it's a distraction and a red herring.
Why? Simple - because no one did
Attempts to present these objections as something other than narrow-minded always rely upon these kinds of hypotheticals, because the case can't be made by actual examples. That's why it's invalid.
One more time:
It's a red herring because it's a false dichotomy - filmmakers don't do it instead of making good movies, and casting non-whites or women in any role has never actually done damage in and of itself to a single film or tv show or play.
Go look at on-line talkbacks and objections to replacing Peter Parker with a black character in Marvel's Ultimates line. Other than complaints about killing off Peter, direct complaints about the move come across as reactive, narrow-minded and/or bigoted. The only posters who think
they're avoiding this are those who resort to what they consider
to be ridiculous hypotheticals, ie "Geez, why don't they just replace Thor with a one-armed octogenarian lesbian?"
The case can't be made based on facts, hence the need to resort continually to "what if they...?"
On the other matter, "respect for source material" does not equal treating it as holy writ in every detail. Frankly, fans who want strangers to spend hundreds of millions of dollars of their money on putting some comic, fifty year-old sf pulp novel or video game up on the big screen but won't allow any adaptation to popular taste, current attitudes or audience preference should just be satisfied with the source material itself and stop expecting studios to risk fortunes on their behalf.