The paper "Are ideas getting harder to find?" might be relevant.

According to the law of logarithmic utility—which has been applied to research funding—a simple rule of thumb is that $1 is worth 1/X times as much if you are X times richer (doubling someone's income is worth the same amount no matter where they start). 

Past the point of increasing returns to scale, the next dollar donated say at the $100M funding mark might have 10x as much impact as the dollar donated after the $1B mark (which is ~basic physics funding in the US— though total physics-related public R&D seems more like $100B and maybe 2x that globally). 

So all else being equal (e.g. personal fit) by switching to a field like AI safety which has even less $100M / y you have that sort of strong multiplier on your side.

These neglectedness/crowdness considerations map onto some fundamental cognitive biases, like scope insensitivity (we aren't very sensitive if a research field receives $1B vs. $10B) and also the sort of counterfactual thinking (people rarely think about the fact that if they don't take that job as a doctor or physics postdoc someone else who's just marginally less qualified will).

Having said that, Einstein is probably a contender of “who did the most good?”, as he revolutionized physics, paved the way for lasers, GPS, and the microelectronics/computers, which in turn transformed modern economies (though some physics might have dual-use risks c.f. nuclear energy and atomic bombs). My read is that Einstein pushed the field forward by many years, so if you think you have strong personal fit for physics, maybe it's worth becoming a physicist.