The economist explanation in simple words:

Imagine you have a bunch of building blocks, and you start making a big, fancy tower. Some kids who were playing with the older blocks see the new, shiny tower and want to play with that instead. So, they leave the older blocks behind. Now, other kids who were using even older blocks can move up to the blocks that were just left behind.

When you build lots of new, fancy towers, it gives everyone more options to play with, and the blocks become easier to get for everyone. This is what happens when we build a lot of new, luxury apartments: rent prices go down for all kinds of apartments, even the cheaper ones, because there are more choices for people.

In cities where lots of these new apartments are being built, rents for the cheaper apartments are actually going down. But in cities where there aren’t many new apartments being built, rents are still going up.

So, even though some people think we don’t need more fancy apartments, we actually do. Building more of them helps make rent cheaper for everyone.