Why run hdmi instead of toslink

Full disclosure. I am not starting a cable war, but I’m really curious why video is routed thru any audio equipment at all. I don’t need atmos or crazy audio. I have a standard 5.1 setup with Khorns and a LaScala for a center speaker routed thru a classe 800 ssp and krell 3250(2). I use toslink out from the monitor in. 
Sound quality is fantastic, so looking for valid reasons to upgrade processor to support the 4K. With 8K out now and I guess 16k coming at some point, does it make sense and to keep upgrading?

thank you in advance.