Monopoly is a bad thing, according to the industry rules. The same may happen to the accounting industry. With more and more scandel in the company they audited, the Big Four don't feel comfortable though they are still enjoying the below amazing statistics:
"According to the General Accounting Office, a congressional watchdog, the “Big Four”—Deloitte & Touche, PricewaterhouseCoopers (PWC), Ernst & Young (E&Y) and KPMG—audit 97% of all public companies in America with sales over $250m. They audit more than 80% of public companies in Japan, two-thirds of those in Canada, all of Britain's 100-biggest public companies and, according to International Accounting Bulletin, they hold over 70% of the European market by fee income.-Economist"