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)
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)