The next 10 years? 0%. We’ll be lucky if the market is flat in the next decade.

10 year treasury bonds are essentially risk free, as they are guaranteed by the US Treasury. Current 10-year yields are a bit less than 2%, so that's essentially your minimum return there.

Assuming that interest rates remain above inflation, which is already no longer true. What's the point of earning 2% a year when the USD is losing 6% a year in value (and that's likely far undercounted).

The point is it's possibly the most secure investment available, so that's why it's a good proxy for the risk free rate.

Whether or not you want to invest at that rate is entirely a separate conversation.

Is that 4% inflation-adjusted?

Usually yes. In times of inflating asset prices, the value of the investment tends to inflate with it.

Usually yes. But … not every year exactly, so it works best when viewed over a longer term.