There's very little serious literature that points to "cheap money" as a source cause of 2008. Neither does banks taking risks because they are "too big to fail".

The main cause you get by reading the research on 2008 is that bank managers were incentivized by their shareholders to take excessive risks. This is because 1) shareholders are pretty shortsighted and 2) managers suffer comparatively little personal consequences by crashing a bank (clearly they don't go to jail; they still get the massive bonuses in the years where they pumped returns with excessive risks, etc.)