B. There are a handful of other good providers out there that hopefully this will give more business to.
Can you elaborate on the reasons for doing so and what motivated that action?
Building it from scratch on top of the WP database made sense for us. The WP commenting code is some of the worst (and oldest) stuff in the entire codex.
I'm pretty sure the first poster was thinking more along the lines of an official company blog, not a major website.
Other than just believing you, what makes it hard? I've had to scale comments on some very high traffic websites and it was pretty straight forward. Unless it's a complex comment system, but then you wouldn't use Disqus anyway.
Now, if you receive a hundred million comments a day, it'll be a little more complicated, but those sites are few and far between. At that point, you most likely have a budget for better hardware and specialized staff.