> Then he can do some pattern matching and try the same pattern in some sites like PayPal, Apple, Gmail, etc. Generalize it a bit and you can even create a tool to do this for you for every new database leaked.
It's the bikelock principle. A bikelock is rarely going to be strong enough to secure your bike. It doesn't have to. It just needs to be secure enough that the thief will nick the next bike. Putting pnt12:HackerNews53cureP4ssw0rd and pnt12@gmail.com:HackerNews53cureP4ssw0rd into every service is going to be profitable enough that they don't necessarily have to try the next step.
But in general you're right: don't use these. Reused passwords aren't secure. Use keys generated by so-called password managers.