That's revisionist and silly[1]. Spending all your liquidity on long term bonds isn't "conservative" if you're a bank. It's not your money! It's your customer's money that you're just holding for them, and you just dropped it all in a vehicle that doesn't mature for 10 years. What if the customers want their money back? Yeah, we just found out.
No, the conservative option if you can't invest it is just to sit on it. That's what banks do. They sit on other people's money. That they are allowed to spend some of that money on speculation is a reasonable optimization, but it's just that. Obviously just sitting on it wasn't going to get the returns they'd promised to investors though. So they placed bets instead.
[1] And I'm not going to touch the idea that the "authorities" somehow forced them into this. No. Just no.