Android's main security issue is systemic and applies to all phones and all versions. If tomorrow morning a new major version of Android fixed every deep seated security issue there was, it'd still be years before it had a sizable marketshare.
I guess this is the answer most users will be looking for. I really like the proposition of the OnePlus 3T and I appreciate the ability to load other ROM's but I'm really just looking for the best baseline secure environment that doesn't require me to either be super vigilant about installing apps or aggressively manage permission on a per-app basis.
