Thing is, a big part of the audience is not following forums, buying merchandise and going to cons. They are people who like the show, follow it every week, then watch something else. Most of these people want something relatable.
Because if there were any Trek characters that the average home viewer couldn't bring themselves to care about, they were Spock, Data, and Worf.
Exactly. There may be a line beyond which people can't relate to an alien being (say, the Horta or Sp. 8472), but the vast majority of Star Trek aliens don't go anywhere near that line.
Not that I have any problem believing the studio heads may be dumb/scared enough to veto the idea based on just such a claim...
The problem I see is one of money. The cost of makeup for the star of the show would put a drain on the budget, since the Captain is pretty much going to be the center of attention and likely in every episode. That brings several hours per day of shooting as extra paid time just to get the actor made up into whatever species they come up with, as most take at least and hour to put the makeup on for a skilled team, and half that to take it off.
Even the most captain centric trek still spent a large chunk of most episodes focusing on other characters (often the same other characters who therefore got pretty much just as much exposure as the captain). Any new show would of course have to balance all sorts of different considerations as to where they want to put their money, but the idea that having an alien captain would automatically be 'too expensive' makes absolutely no sense to me whatsoever.
My biggest problem with an alien captain would be how to strike the right balance. Make them too human like and you loose the entire benefit of having an alien captain in the first place. But the more un-human they are, the more time you have to spend explaining what their background is, what their culture is like and why they act the way they do.
And while I don't believe the Captain needs to be more relatable than all the other characters (and good Trek show should have at least 3 or 4 major characters from the start), I do think any standard formula Trek series tends to rely on the Captain to set the tone for the series early, which would be a very hard line to walk with an alien captain, since they have so much extra setup necessary to help people understand them.
Unless you throw out the standard Trek formula and do
something different, in which case, there's absolutely no reason why they couldn't do it.