In the TNG episode Tapestry, we learn through the alternate timeline provided by Q that Picard becomes an assistant astrophysics officer. Why would this be though? Wouldn't Picard become a historian like we see those who infrequently pop up in several TOS episodes, as that's his passion?
It's not made clear whether Picard would really have ended up as an Astrophysics officer had he avoided taking risks in his life or whether Q has simply chosen to place him into that role in order to hammer. his. point. home.
There's a quote in Voyager regarding the abilities of the other Q known as Quinn;
CHAKOTAY: How about Stellar Cartography?
JANEWAY: We could shut down Stellar Cartography with all the knowledge he'd bring to the job
My personal theory is that given that the average Q knows everything about astrophysics, Q may well have simply chosen a profession for Picard that he personally considers to be utterly worthless.
Regardless, the decision to leave Starfleet and become a professional historian or archaeologist would still require him to take risks. It's made clear in quite a few Trek episodes that alien archaeology regularly results in uncovering dangerous artefacts so it's unlikely that a fully risk-averse Picard would have gone down that route.
Q wanted to highlight that due to Picard taking these risks (early in his life), he got to be as successful as he is. If that would not be in his character, he would have ended being quite unsuccessful in his life. Which, as in the example Q makes, would be assistant astrophysicist with no real chance of promotion.