although with Steam, the modding scene will only be a shadow of its former self rendering ArmA 3 little more than a good game rather than the incredible piece of software its predecessors were.
That is an asinine statement. The only games that Steam affected negatively for modding communities were from when Steam was first released. If that is the only reason why you'll leave, then you're making a very misinformed decision.
Skyrim's modding community is just as big as Oblivion and Morrowind even though it uses Steamworks.