Not “everyone”. Imagine the economy like a tank of water, and each person a single molecule of H20, and proximity to the top of the tank represents relative wealth. So all the water molecules are constantly shifting a bit, but mostly stay in place unless something shakes up the tank.

If you sprinkle water on the top, those molecules stay mostly at the top: rich parents’ kids mostly stay rich. If you feed water in through a pump at the bottom, most of those molecules will stay at the bottom: poor parents’ kids usually stay poor.

In general, the closer you are to the top or bottom, the more likely you are to stay there. Adding from the bottom pushes the people at the top even higher, yes; but those in between the bottom and top mostly just shift around as the middle gets wider.

It's more like a pyramid, where the people at the top of the pyramid go farther up the more people you add to the bottom. The size of the whole pyramid increases over time, and some people might move up or down a few levels, but the majority of people will spend their whole lives near the bottom.

You still have innovation in a stable population size.

What's "innovation" these days? More ads? An iPhone with 5 cameras? A new SaaS product that's a glorified spreadsheet/database?

That's not where per capita growth comes from...

Most per-capita growth is just inflation. Additionally, metrics like CPI don't include inflation of assets like real estate, even though the utility of a home doesn't change over time without some kind of upgrades. Real estate in particular is inflated largely because of the ease of acquiring leverage.