My assertion that "Given enough time, this most likely *will* happen" stands by the law of probability that given enough time/opportunity even unlikely events become probable. Even if the likelihood of malware for macs is low, with every day the chances of it happening increases.
I am also claiming that the inspiration for malware writers to target Macs may not be as small as you would think. Apple currently has a non-negligible number of installed users, so even if the percentage of total users is low, Apple provides a significant number of targets to malware writers which to this point have been overlooked.
As far as the security concerns with local admin rights, I consider allowing users to have local admin rights in an enterprise setting to be an implementation flaw. Microsoft implementations do not need to have users as local admins (and neither do Mac implementations). The local admin rights under Microsoft do not only grant the permission to install software, but also modify the OS files, security settings, manage users/passwords, etc. Giving users the ability to make these types of significant changes to their own machine, regardless of platform (or use of sudo or direct access), can only lead to weakened stability and security.
To sum up, chances are Mac users will be a target, this eventuality must be planned for, and lessening user privileges (taking away local admin rights) is one way in which security can be improved and this threat partially mitigated.