If, if, if.
But the discussion is not about hypotheticals but about facts. And the one you keep disputing is the commonplace statement that many of the banks and other institutions brought down by derivatives lacked the competence to assess the risks of these opaque products.
The story is of sleepy, highly regulated institutions who suddenly changed their business models when derivatives/banking were deregulated and entered the game - the level playing fields

- without the internal governance systems or institutional expertise to deal with the risks they were exposing themselves (and their customers) to.
Now you offer cute annecdotes about car yards. We are talking about banks where hiring a few traders is not good enough. There has to be a whole culture for a bank to be an "expert".
You mentioned earlier the S&L saga in the US. Perhaps, like Marco says, you just lack familiarity with how similar the parallels are regarding European banking and structured finance in the mid-2000s.
But it has been much written about. For example, LSE's Robert Ward on the Icelandic banks...
Or the Economist on Germany's landesbanks...
A Forbes comment at the time...
The reason all this matters is that without the steady flow of suckers, there couldn't have been the crisis. Goldman Sachs and the other big city sharks needed someone fresh off the bus to buy their shonky wares.
You can keep protesting that these hick, inexperienced, unready financial institutions were technically "expert" and it was quite legal to relieve them of their small town innocence as part of their rapid and necessary life education having arrived in the big smoke.
But I'm not sure how that squares with you also feeling that the derivatives world is much in need of regulation to protect these same "expert traders".