"Reversion to mean" - most often used in stocks and financial discussions. Is it more universal? Does everything ultimately "revert to mean"?

It is generally applicable to any data set over time. But the mean can move.

The idea is universal. Suppose you flip a coin and get 5 heads in a row. If you keep flipping, the percentage of heads will move towards 50%.

I think it depends. It does seem like it is generally true for many things, although the trend matters.

Also, some things seem to go back and forth between extremes rather than hover around a “mean” or what I would call a more moderate position. I call this the pendulum swinging. In politics we see everyone talk in extremes, like the country can only be capitalistic, or socialist, but really we have a kind of in-between.

What about someone who has a boyfriend who is extreme in some behaviors, and after a break-up they next date someone very opposite regarding those extreme behaviors is that going back to the mean or is that going between extremes? If the pattern is the person keeps getting attracted to the behavior, even if they complain about it and don’t like, then that is their average I guess.

It’s a statistical idea that applies to anything where it’s valid to say that the thing measured can be expected to vary but has a fundamental value that doesn’t change.

Not everything behaves that way.

Some things do change, even in the movement patterns of the values.

It’s an expected pattern in economics, but values definitely do change. If someone says a stock value will return to mean, they’re asserting that the current price likely to revert to where it was more often in the past. But it’s an assertion based on the notion things will tend to stay as they were.

Another possible translation in economic contexts is “the value has changed from what it’s tended to be, and we don’t know of a reason for that.” But there are many things people (especially economists) don’t know.