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.
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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.