True enough. Other than observing the different cabin interior design (I have flown 737s with open overhead storage and the breadbox oxygen mask, ventilation and reading light housings), it's all the same unless you have some degree of luxury seating (probably only in the 737s operated by Privatair for KL among others).

The only other attraction of flying an early 737 is using the self-stowing stairs that these a/c could be equipped with if you get to a hardstand airport (sort of like using the tail cone stair of a 727 or DC-9, not sure if the M80s every came with these).

I'm one of those who has flown on too many 737-200s (including the only WS -200C, in all-pax configuration), too many -300s and -400s (handful of -500s on LH and CO), and a smaller amount of -700s, -800s and -900s (as well as the sole -600). Only one Max flight so far and that's on a MH 7M8. Hard to tell them apart other than the cabin interior.