Hm, well, it would probably just affect an arc or so. I don't really think you have to like the main characters to enjoy a manga for reasons stated earlier. Natsu and Lucy for example though main characters also share similar panel times with the likes of Gray and so. This is also the hangout thread so feel free. There's no need to apologize.
But Gray and Erza are the deuteragonist and tritagonist of this story. So it makes sense that one of them could take a lead role at any moment in the manga if lets say Natsu went evil for a arc. But it seems like a few fans of fairytail would rather see Gildarts or Jellal take on Zeref and not see Natsu surpass Zeref using his own power, or wish a character like Mirajane some how take what ever limited role Erza has.
While it's understandable that most fans just want to see their favorite character take a more active role, it boggles my mind that some would just rather a side character just some how replace one of the lead 4 in importance to the story.
Plot wise one of the 4 main characters will always have some type of special reason why the manga focus on them.
Maybe Hiro Mashima just didn't do a good job portraying Natsu and Lucy to his readers. You don't really hear to many fans complain about Luffy or Goku saving the day as much as they complain about Natsu.
But to be honest Natsu didn't interest me till his backstory with Zeref came to light, I was always a Lucy fan right out the gate.