(Own work; adapted scaled drawings via boeing.com)
It's no coincidence the fuselage widths match, the 757 borrowed the 727's fuselage, which width-wise is the same as the 707 and 737.
@Brilsmurfffje and @CarloFelicione already raised good practical points, namely the indirect airline costs and why airlines would favor the 737 over a 757 re-engine, and the reboot costs.
If we look at the two planes from a pure engineering viewpoint, the 757 makes for a lousy substitute.
The MTOW is 27 tonnes heavier, but we'll gloss over that and say Boeing will certify it at a lower weight.
Bad wing (for the mission)
The 757 as shown above has a lot more wing. The wing areas are 185.25 $m^2$ and 127 $m^2$, for the 757 and Max, respectively. (45% more wing.)
Bigger wing is higher empty weight, i.e., the plane has more material, and subsequently the associated manufacturing costs. 13 tonnes of more stuff not counting the re-engine – newer engines are heavier. As for using CFM LEAP, between the highest 737 Max thrust and lowest 757 thrust option, the 757 has 13 kN more, so we're okay there.
Those 13 tonnes at approx. \$1,700/kg OEW:unit cost correlation already make the 757 some 22 million USD more expensive without taking into account the development cost associated with a re-engine. Unlike MTOW, there is no workaround for that.
Secondly, the 757's bigger wing is really bad for the cruise economy of the 737 missions. Low wing-loading is good for maneuvering and for using short runways, but cruise economy comes from aspect ratio and – ideally if it won't impact too much the takeoff performance – high wing-loading.
The 757 needed such a wing for fuel carrying reasons (volume and MTOW), after all it's suited for transatlantic operations using older engine technology. But when that much fuel is no longer needed (re-engine), the physical big wing remains.
Now, much like how the A330neo complements the A350, one could argue a re-engined 767 could complement the 787, but like also how the A330neo ended up badly for the A350-800, a 767 re-engine could be troublesome for the smaller 787s.