I think people misunderstand how hackers behave. Just because there's dev mode, doesn't mean hackers won't try to break into the system. I'd say they'd do it in spite if Dev mode. Dev mode itself is heavily restricted. No hacker worth their salt will just back down because of that
PS3 had Linux mode officially supported in early models. Hackers still hacked their way all over it that Sony discontinued the service.
The Xbox one is secure, because this isn't Microsoft's first time dealing with their systems being hacked, both Xbox and Windows