Default [Q] Method for rooting Xperia with unlocked bootloader

Hi!

So finally my old phone gave up and I decided to go for Xperia SP thinking I would flash Cyanogenmod on it. Now that I have used it for a while I decided I'd keep the stock but still unlock and root the phone to be able to restore data folders (WhatsApp!).

Now most of the rooting methods here are workarounds for locked boot loader models and involve finding the right firmware from Sony and a lot of hassle with the file patching. Even with unlocked models the rooting works by flashing the exploited kernel and using the exploit to gain root.

My question is why is it not possible to just boot (NOT FLASH) modified kernel with security off via fastboot and then upload SuperSU or Superuser files into device and then after installation continue using the kernel provided by the firmware? This is also called the general method.

I will go this route if there are no reasons not to.

PS. Thanks for this wonderful community. As a long time lurker I finally decided to join you!