The people who buy a book
purely because it is known, popular, and 'in the public consciousness' enough that everyone knows about it, aren't the sort to browse for new authors.
I highly doubt that the sudden appearance of the something like 50 Shades of Grey (for an obvious example) will have had any impact on the sales of a new and aspiring author, quite simply because it became popular
because of the huge coverage. If that hadn't happened, the millions of people who bought it wouldn't have been roaming bookshops looking for a good read, they would have been carrying on with their normal lives, just as oblivious of the charms of a good book as they were the week before.
Rowling is obviously a slightly different matter, but the end result is probably better for authors as a whole.
The first book isn't great (personal opinion only!
) but it's definitely readable and entertaining enough to read and enjoy,
especially for the younger audience that it is targeted at. If that, and it's subsequent explosion of publicity, brought in thousands of people who would never think to roam the bookshop for a story, then great! When they put down that book, there's a much greater chance of them looking for another similar good read than there is them just going "okay, that's done." and going back to not reading again.
As Bowler1 says, if you happen to get published the same week that "Harry Potter: the next generation" gets released... well... ****... that's some pretty awful luck, but that's no more Rowling's fault than it would be the fault of a postman who delivers bad news in the mail. She might not have the same desperate need to earn a living that an aspiring new author does, but she has exactly the same right to write the stories she wants to tell.
In short, Lynn Shepherd in the HP (irony...) is probably just feeling entirely natural and understandable jealousy, but chose to lash out publicly online. Which is likely to make her look a right tool, and lose her a lot more readers than the next JK Rowling release will!
(Of course, it might also be that it's a calculated attempt to generate publicity for herself, get her name out there, and increase her future sales... they DO say there's no such thing as bad publicity!)