One Bcash wallet owned by Bitmain with a staggering 440,000 Bcash in it, that bag gets bigger everyday because nobody is buying that shitcoin. 

It'd probably be a lot bigger if the largest Bitcoin holders (like the WinkleVii for example) would dump all their remaining BCash, but I imagine that some of them just can't be bothered to jump through the hoops.

How do we know that Winklvii and some of the other BIG ones have not dumped their Bcash?
Well I guess we don't know that, it would take some sleuthing to know for sure. The WinkleVii claim to have not sold anything and have all their Bitcoin locked up.
But then with a BCash major dump the question is who would buy it? The BCash float is so tiny and thin right now that it couldn't take a major market dump. Not on the scale of what the Bitcoin market can absorb.
I have heard some of the Windlevii claims, and I just don't buy the parts about the locked up bitcoin. I believe that they can honestly maintain their claim that they have not spent any of their bitcoin, but still move their bitcoin. It seems that they did use some of their own bitcoin to collateralize petitions that they were making.
I do agree with your point that there are likely some BIG OG Bcash holders out there that currently could pretty much drive the Bcash market into a pretty low price status, if they were to chose to take such action. It may be wishful thinking to hope for such, and perhaps we are merely going to witness ongoing Bcash bleeding.. and maybe some here and there attempts at pumps. Unless we witness some pretty calamitous Bcash price dumping, I don't doubt that there are some businesses that might not be gullible enough to take bcash on as a "cheaper bitcoin," which could cause further pumps.
with bitcoin's volatility, it is difficult to borrow against it (in Winkelevi case); perhaps they used or will use futures.
I am not saying that they actually borrowed against it, but when they filed their various licensing petitions, whether with the exchange or with the ETF or other business branches that they have, they likely used some or part of their bitcoin stash as a form of collateral to show that they could support some of the business liquidity.