Is The Silent Expanse The Aftermaths Of The Decay

is northern west gavel slowly being turned into silent expanse like terrain from the decay or will it just die and thats it?

 

It'll just die, the way it works is that the Decay is a result of light being sucked out of an area. The SE corruption is just the same as the Roots of Corruption except SE was never touched by light and thus is darker

 

Both the dark/kander forests and SE are being affected by darkness, although, SE is being affected by it more directly as it has direct access to Dern via the portal, while the dark/kander forests are only being affected by the parasite and dernmites. The main parasite seems to be powerful enough to turn small areas (or large areas, like the Heart of Decay, over a long period of time) into a somewhat SE-like terrain, although the dernmites seem to only be powerful enough to kill things and leech the light.
So, no, I don't think it would be turned into SE-like terrain even if the dernmites weren't stopped, although if the parasite had remained there it possibly could have, after quite a long time though.

SE darkness and the corruption are not at all the same. Light and dark are powerful forces, corruption is created when they clash. It's not a spectrum or something, you can't "take away" light or darkness from corruption and somehow get the other one, you can only create corruption by combining them. (Also, SE was touched by light via the Lutho monolith.)