There's absolutely no way Arrow gets cancelled any time soon. It's going to go 8-13+ seasons easily like Smallville, Supernatural, and The Vampire Diaries.
Vampire Diaries lost its lead character, got shipped off to Fridays, and is doing less than half of the ratings it did in its prime, and it's still probably going to get renewed for an 8th season.
Right now Flash is the highest rated show on the CW by a huge margin. Arrow and Legends of Tomorrow are tied for second, and the next closest show they have is Supernatural, which does about 75% of the ratings those two shows do. Nothing else on the network is even close to Supernatural.
Also, Arrow is one of only a small handful of shows across all of television that is actually up in the ratings versus last year. TV across the board is down 11% in the demo this year versus last, but Arrow is + ~2%. And the third season was higher than the second season. The other Arrowverse shows have really lifted it (specifically Flash; the crossover episodes of Flash are Arrow's two highest rated episodes ever, but the Arrow crossover episode of Flash this year was rated below the show's average). Next season brings the possibility of even more crossovers with Legends and Supergirl, plus possible return appearances by Constantine and Vixen.
CW will ride Arrow for as many seasons as they possibly can. More than any other network, the CW relies on Netflix/Hulu streaming, digital sales, international sales, DVD/Blu Ray, and syndication. All of those alternative revenue streams are tied to how many episodes the show has. As long as the ratings are anywhere close to decent, Arrow will be renewed. It's already got four seasons under its belt, which gives it a huge advantage over newer shows.