Why do people say that, when the number one hardest problem is making good abstractions?

Because it’s a “famous” (in our circles) quote. You might prefer this one:

> There’s two hard problems in computer science: We only have one joke and it's not funny.

There are at least one more joke:

"There is 10 kinds of people, those who can read binary and those who can't."

Personally I prefer the cache invalidation one.

> "There is 10 kinds of people, those who can read binary and those who can't."

I like the continuation (which requires knowledge of the original): “And those who didn’t expect this joke to be in base 3”.

Names abstract things.

You can do the very same with inheritance, where `Player` and `Monster` inherit `Flying`

What about walking monsters? Another inheritance branch for those? And other subtrees for other effects? What about making those effects temporary?

RegioJet is a much better and cheaper service than the state-operated rail service.

It's not the only one. Czech private rail companies are also a huge success.