so you're saying its because I don't have quality backlinks? But it was fine all this time and they have been sending me traffic this happened all of a sudden and not gradually. One minute there was 30 from google real time and next minute 2. That's what is confusing.
Last year on a different site when I had the exact same problem I thought if I redirected to a new domain it would fix the problem and it did for about a week I got all my rankings back and after a week I lost them again.
Could be that you being fine 'all this time' just means that Google's algorithms hadn't flagged you for anything spammy/copywrity/etc and this 'sudden' drop is them just finally clicking it on.
If it were me, I'd take a deeep breath and give it a few days. It sounds systemic, but there are a lot of unknown unknowns out there. After that, try to find the most recent backlinks pointing to your site (did you get a report from services purchased on BHW?) and assess their potential for negative impact.
Check new links, your competition may have just blasted a lot of bs new links your way (it does happen)
I have had sites rank page 2-3 for keywords, drop OFF the top 100, and then come page page 1-2. Given, that's for specific pages and not a site-wide phenomena.
Still sucks though, I'm imagining that you have some programmatic method by which you find/get/post content to that site (you said 400K indexed pages) ?
Another option would be to just transfer the content to another domain, and try and 301 the links you
know aren't penalizing you to your new domain. Something that I've found helpful is the purchasing of expiring domains that still register as being quite old. Since the metrics don't need to be PBN level, you can usually find one with a sem-relevant name for < $20. Obviously, if you're trying to use the keyword in your domain name this doesn't work (unless you get reeeally lucky)
Anyway, good luck. Hope it's just something odd that resolves itself.
EDIT:
Also, if you go the route of content transfer, you could probably just 301 your entire old domain to the new one. The 'juice' gets passed, but G sees it as a single link. Doubt they'd penalize you for a single link.