Also - couldn't you actually just sign the drivers that are needed for trim? What prevents that?
As the author of the popular "trim enabler" software (which patches the original apple drivers and so causes the original drivers to fail the kext signing check) puts it:
"all of Apple’s AHCI SATA drivers are closed source and undocumented, which makes it impossible for me to create my own Trim driver and get it signed."
Which is also the reason why there are no trim drivers available from hardware manufacturers like Samsung, etc. No access to Apple's driver documentation - no signed trim drivers.