One important fact left out is that the credit agencies are in the business of grading securities. Companies like Goldman Sachs (GS) are in the business of creating securities. So if one agency grades all of GS products as garbage, they'll just use the other two agencies to grade. If you suddenly stop grading all of GS products, that's 10's of millions or hundreds of millions of lost income. Or you just give it whatever grade GS wants, and effectively become a rubber stamp company.
OP has a nice edit in there explaining how he's not giving the banks a free pass, but at a certain level the banks were knowingly (or maybe even unwittingly) perpetrating a fraud. They saw the rules and gamed the system so they could maximize their profits and bonuses. The banks knew they were selling toxic assets, too, because The Big Short (book or movie, your choice) shows the cost to insure Mortgage-Backed Securities or Collateralized Debt Obligations through these banks went waaaaaay up while they were still selling the same products as if they were worth full cost.
If I have to assign blame between someone who is negligent and someone who is fraudulent, the fraudsters will get the majority of it the whole time.
I worked in Dodd Frank Regulation as a "model validator" for two years. The credit agencies are responsible, true. But I think the SEC should do their own ratings and hire competent people (and pay them well enough to keep them). We do stress tests (CCAR and DFAST) that honestly, are kindof a joke. By that I mean, the regulators do dumb stuff too. In my ideal world the SEC would rate banks and then let the market digest that. Taking on too much risk? Fine let stock prices adjust.
The problem with having the Fed do stress tests is, in my experience, the best quants end up in the private sector. We had two rather bad quants that got fired and they both ended up at Fed. In one review of my work I actually had a Fed lawyer question me. His dispute? "How come last year when you reviewed this model there were more Findings?"
Let me explain. We review models. We test them, try to break them and find fail points or unaddressed weaknesses. We make findings and enter them into a document. Model validators are under intense pressure from our managers and the Fed to maximize Findings as a show that we did work because managment cant really do quant work so page count and finding count of our reports was how they measured success.
Result: a typical validation report would be 160 pages of redundant crap with 30-40 findings. More often than not, when I read others reports, they would miss the main model flaw but point out grammatical errors in documentation. As a result of the Dodd Frank act there was a quant hiring boom and they really scraped the bottom of the barrel in regulation.
On the other hand, I cant blame Moodys. This isnt accounting fraud where you can tell numbers don't add up or are hidden. A common problem we saw in models is usage of "non stationary" variables to predict default or loss given default. They often used automatic variable selection methods and the auto methods didn't care about this.
This matters because if you are predicting default and using say, GDP as a predictor, you will magically predict default rates to go down purely as an indirect function of time (GDP generally always goes up).
We tend to think bankers at the top were plotting some evil stuff but in reality they figures their nerdy quants knew what they were doing and weren't equipped to judge their work and the banks didn't have effective internal challenge.
The one thing that the OP touched on, but did not emphasize nearly enough, is that the reason the bundling to create "A" rated securities was done was because of the massive demand for high-yield-A-rated securities from pension funds (particularly state pension funds). These funds like CALPERS are significantly underfunded, and assume overly optimistic rates of return (7.5% annually for CALPERS). The only way to get that good of a rate of return is with a risky investment, but state legislatures (smartly IMO) prevent pensions from investing large percentages of their portfolios in risky stocks/bonds (so mostly they only invest in blue chip stocks with dividends and municipal securities). Mortgage backed securities are another, traditional, blue chip investment (because most people pay back their mortgages), however, they don't pay 8%+ interest, but risky loans can (but were never rated as 'A's by the ratings agencies). Thus, companies bundled these risky loans with good loans to satisfy their clients' desire for high-yield, "safe" investments.
There's plenty of blame to go around for the financial crisis so I just wanted to add that AIG were probably the stupidest motherfuckers on the planet for insuring all those garbage loans.
Basically one of the big reasons for why the credit rating agencies were able to certify subprime mortgages as AAA was because the insurance company, AIG, was rated AAA and sold "insurance" on the mortgages in the form of credit default swaps. Everybody was paying premiums to AIG so that if some of these loans went bad AIG would take the hit and pay out just like ordinary home/car insurance. With a AAA insurer backing the mortgages the credit ratings agencies gave them a AAA too. The great thing about credit default swaps as opposed to ordinary insurance is that they're a form of derivative so they're totally unregulated (thanks Bill Clinton) and confidential so AIG never had to disclose the mountain of risk they were actually taking on and could keep that shiny AAA rating. So AIG just went on gold stamping any pile of shit they found while raking in millions in premiums for exposure that properly priced should have been in the billions.
Of course when the shit hit the fan and AIG was on the hook for hundreds of billions the government swooped in and bailed them out, insisted that they pay 100 cents on the dollar on their credit default swaps (with the governments money) and they even paid the dumbfucks who made these deals their contractually obligated bonuses so they could keep their 'expertise'. Can you imagine fucking up that bad and keeping your job? Without these guys backing all the CDOs the financial crisis might have never happened.