Would you ever be surprised if an old car got brand new tires? No? Then why do you find it so surprising that engine manufacturers would build new engines for existing airliner designs?
That's more like fitting oversized wheels / tires that will rub into the well / body every time you hit some proper bumps. Sooner or later they will fail, spectacularly.
Tires, and a powerplant are two different domains. There's lots more at play with structure & what other parts can withstand. Resonance, materials, & aerodynamics all play a factor in the design process. Combine it with flying through the air in a seat instead of the ground, makes it all even more a factor.
Ever see Mazda Miatas with a Chevy LS motor? Kits are sold to adapt, especially now everything is drive by wire / software.
I understand your point, but this is the reality of modern aviation. New engines are released for existing airframes. In this case though, it wasn't an existing airframe. This was a new model built around new engines. Where the analogy starts to break down is that unlike putting new tires on, any face-lift that is done to an airliner is backed by testing and and under the watchful eye of multitudes of regulators.
I would certainly be surprised if after installing the new tires you are required to install a new system to compensate the brake or you car may crash. The MCAS system is a clear indicator that the airframe and the engine are not compatible and yet they decided to do so for profit.
Would you ever be surprised if an old car got brand new tires? No? Then why do you find it so surprising that engine manufacturers would build new engines for existing airliner designs?