OS vulnerabilities in 2005
Statistics show that during 2005 the following number of vulnerabilities were found in the operating systems:
812 Windows operating system vulnerabilities;
2328 Unix/Linux operating vulnerabilities;
Furthermore, since Windows dominates the desktop OS, I tend to believe that most of the possible exploits and vulnerabilities were already discovered. However, since Linux has only a small share of the desktop, it was not stressed enough for all the vulnerabilities to be discovered.
In conclusion, if Linux had the market share, it would be the main target for exploits and attacks and many more vulnerabilities would then be discovered.