There is absolutely zero chance this is the show's last season.
1. the main cast is signed for seven seasons
2. the show has a per-episode payment from Netflix and a brand new syndication deal on TNT, to say nothing of international sales and DVD/Blu Ray/digital sales. All of those factors mean there is a ton of financial incentive to keep ordering new seasons.
3. even hitting a series low 0.7 rating for Arrow is still going to be better than any other non-DC show on the network. The only thing even in the neighborhood would be the ageless Supernatural.
4. the producers have explicitly stated that this will be the last season of flashbacks... to make that distinction implies a plan of doing further seasons without flashbacks
Unless something horrible happens to one of the main cast or producers (of course we all hope nothing does), this show is going seven seasons minimum. Probably more, looking at how far they've dragged out similarly successful shows (Smallville, Supernatural, Vampire Diaries).