In my case, usually something is broken or breaking in production frequently enough to warrent some changes. Plus, there are other reasons you can make a change even though it's not broken.

Sometimes it can make you more productive. Or though your site is still responding to current customer demands in a timely fashion, you know that the mobile experience could be significantly improved now that browsing via cell phone is on the rise.

Another thing to consider is employability both from a company and individual perspective. If you can keep up with moderately current (not the latest and greatest) trends, you'll attract people who want to grow in their careers. I wouldn't want to work on C# 2.0 using Visual Source Safe. It's hard to convince a company that you can learn git on the job.

In general I like to move without introducing breaking changes. I'm not a cowboy coder, it's really exhausting working with one. I do think there's merit in realizing when it's time to change though.