If Windows could work without SMM, is there a historical reason why SMM mode didn't just die and become disused after Windows becomes popular and nobody uses DOS any more? There are plenty of features in x86 that are disused.

The feature turned out too useful for all sorts of things, including dealing with the fact that before NT loaded itself you still had to emulate being an IBM PC including the fiction of booting from cassette tape or jumping to ROM BASIC.

Also, it's been cheaper to implement various features through small piece of code instead of adding a separate MCU to handle them, including prosaic things like handling NVRAM storage for variables (instead of interacting with external MCU or having separate NVRAM, you end up with SMM code being "trusted" to update the homogenous flash chip that contains both NVRAM and boot code)

> Thus, SMM, because there was no other way to hook power management on a 386 laptop running " normal" DOS

In theory, there was: you could have a separate microcontroller, accessed through some of the I/O ports, doing the power management; it's mostly how it's done nowadays, with the EC (Embedded Controller) on laptops (and nowadays there's also the PSP or ME, which is a separate processor core doing startup and power management for the main CPU cores). But back then, it would also be more expensive (a whole other chip) than simply adding an extra mode to the single CPU core (multiple cores back then usually required multiple CPU chips).

The problem is reliably interrupting the CPU in a way that didn't require extra OS support. SMM provided such trigger, and in fact is generally used as part of the scheme with EC cooperating.