It’s been stated rather clearly, hasn’t it?
Of course, if you still miss some legendary mercs and the quest reward rolls for them, you might get a base portrait in such cases (I think there’ve been some reports of that in the references or somewhere else), although the odds for it are quite low if you own most of them.
It’s been a while, and so far we haven’t been communicated ‘officially’ about the matter, whether there is an issue and so on. Your assumption of what to expect in the future is as good as mine.
If you’re interested in my reasoning and the choice I’ve made about these story quests, more about it: Legendary merc reward: why it had to be Jaraxxus? .