I've written another article in this series on the Christ myth theory, and it goes into more detail on Ehrman and the appeal to the authority of historians.
I'll just point out that Ehrman's an exception that proves the rule. Most scholars who specialize in the New Testament are Christians. Even atheist NT scholars like Ehrman and Robert Price are ex-Christians. Indeed, they're ex-fundamentalists.
I agree that atheists can have axes to grind too. But it's our formative years that define us the most. It's not so strange, then, to think that Ehrman's former fundamentalism clouds his judgment on some fundamental questions about Christianity. There's also the possibility that he's arbitrarily restraining his skepticism so he's not perceived as a rabid atheist, with the pendulum merely swinging far in the opposite direction.
Also, not all atheists are antagonistic specifically towards Christianity. And outsiders often aren't susceptible to the same group-think to which insiders are prone.
The appeal to God in explaining the historical success of Christianity is superfluous and idle. It's a God of the gaps. Also, the Jesus of the gospels would have repudiated the version of Christianity that triumphed in the West since it expects far too little of its members. But that's why it triumphed as the face of a desperate, collapsing, transactional Roman Empire.
What's the "error" that you think the Gnostics committed? Do you think theology is as rigorous and as airtight as math?
You think it's unlikely that corruption rises to the top of society until it can consolidate its power and dominate the masses for generations. I think your assessment leaves out the fact that that was the norm in all ancient monarchies and empires.
Or just look at the modern arts: there are the elite arts that are praised by critics and that don't sell well at all because they're too challenging, and then there are the kitsch and the trash merchandises that are bland enough to have broad, "catholic" appeal.
Look at the kinds of articles that succeed the most on Medium. Are they the highest-quality ones or do they merely pander, defraud, and dumb things down for readers who prefer to be entertained than challenged or educated? There are a few exceptions, but again they're exceptions that prove the rule.
I certainly think you have things backwards here. You're trying to make room for a miracle of Christianity's success in history, but in doing so you're overlooking the global historic and societal trends. Indeed, you're overlooking what you should be calling the fallenness of "natural" people.