But even if Apple may be better than most android manufacturers, Apple still doesn't support devices it considers "Obsolete"...and there is nothing you can do about it other than buy a new phone since the bootloader is locked down. According to https://en.wikipedia.org/wiki/List_of_iOS_devices anything before iPhone 5 is considered "Oboslete" and they don't seem to be releasing updates. On the other hand, I've been happily using my Samsung S3 which cyanogenmod has been pushing latest android updates to nightly...and that is likely to continue as long as there are enough of us who care, even if cyanogen and samsung goes away.

For the record, the Galaxy S III was released in the same year as the iPhone 5 (2012), so for now your device hasn't lasted any longer than an iPhone. But I don't doubt that it will.

SII last got an update a month and a half ago (https://download.cyanogenmod.org/?device=i777)

Without any updates for the vendor blobs.

Does the S3 get security updates to all the kernel-level vendor-provided blobs?

cyanogen is no more.