At present the problem with not allowing Hawkeye to automatically correct a wrong call is the cost of installation. Many articles suggest the cost is $60,000-$100,000 per court. The price will go down as the technology improves but it is too high to be installed at every place as of now.
Another thing associated with cost is the likelihood of technical problems or electronics failures, and backups would add on that cost.
There are other issues that make it unlikely that linesmen would be replaced altogether.
Geoff Pollard said "We might have electronics failure," (if that happened) where do you find 350 linesmen at short notice?"
Paul Hawkins of Hawkeye Innovations said he believed the ITF was looking only to use the machines for replays of disputed points.
"I think that's what they want," he said. "I think with machines deciding every point it would become too sterile."
Blockquote of Pollard and Hawkins was obtained from The Age article by Richard Hinds in source given below.
As of now only bigger tournaments are able to employed this technology even with backup but that doesn't justify scrapping a linesman altogether. But we can't neglect the idea that this won't happen in future, when price is justified.
Sources used for this answer: The Age, Reuters, USA Today, The Atlantic, Sports Illustrated, The Guardian and rediff.