Michael pretty much cast his fate in concrete when he agreed to take over the reins from Vito. Now the other Dons saw him as foe, not just a one-time defender of his old man. He put himself in the kill-or-be-killed position with the other Dons by becoming a Don.

He couldn't simply have walked away from everything when he moved to Nevada. First, he'd made a lot of enemies as a result of the massacre, and they'd come for vengeance unless he had muscle (Clemenza and later Frankie) to protect him. He also needed that muscle to protect hiom as he built and expanded his legal gaming empire--without it, as you say, real gangsters would otherwise take what was his.