What is the worst thing that will happen if I do not update iTunes to its latest version?
The only way to know this is to watch the release notes for every subsequent version of iTunes and see if the release has any security content.

This will change over time, but the present version of iTunes for Windows is 12.1.2, for which the release notes can be found here: https://support.apple.com/kb/DL1614.

You will note that security is not mentioned for this release, but you do need to be aware of future releases so that you can check their notes when they come out to see if any flaws have arose.

You may find it easier instead to just follow some Mac news blogs, since they typically all run stories when an update is released.

Overall, however, I wouldn't say this is a tremendous security risk.