macrocephalopod

Most trades that "worked" have some combination of risk premium/structural alpha, and genuine alpha. That is, you were getting paid to either take a risk or perform a service, but also you got paid more than you "should" have because others had not noticed how good the trade was

Equity stat arb (i.e. cross-sectional price reversion) is an example of this. This provides the service of supplying liquidity to impatient traders and it therefore has positive expected return. But for many years the positive expected return was *way* higher than necessary

to induce traders to provide the service, because not many had noticed you could do it (or had the necessary infra). So it had genuine alpha in addition to structural alpha. Now it only has structural alpha and is less well rewarded as a result.

I'd argue most forms of quant value and trend following have a similar property. Value is a risk premium and trend following (on average) provides the service of taking on price risk from hedgers, but they both also had had genuine alpha in the past which is largely

absent today. For these strategies in particular past performance is not a guide to future returns.