Just because you never had a problem doesn't mean it's fine. Or safe. I've never been in a motorcycle accident despite 20+ years of riding around the world yet I would never claim that riding a motorcycle is perfectly safe and nothing will ever happen to anyone simply because I've never had a problem 😉
What happens when you're running an unsupported operating system can be quite tricky to assess. Chances are you'll be fine as long as you keep your web browser, email client, and other applications you use on a day-to-day basis to communicate with the outside world (file transfer clients, chat and communications software, etc.) up to date. Problems will arise once a serious bug is found in macOS 10.13 that gets exploited. Again, chances are this will never happen because given it's effectively negligible niche status macOS is not a target worthy of serious consideration and effort. Thus, statisically and despite all those repeated claims by security software manufacturers coming up with these grim and menacing studies of how much more dangerous it has become to run macOS without their specific flavor of anti-malware protection year after year after year there's a very very high chance that you'll be safe for many years.
However, security by obscurity is not the greatest way to protect yourself against harm, and therefore the chances of your data getting stolen or encrypted are higher on an unsupported system than on a supported system. That's the bottom line.