Via Matt O’Brien, this chart from JP Morgan shows financial sector leverage over the past few decades. As you can see, leverage skyrocketed during the Bush era, which contributed to the 2008 financial meltdown, and then plummeted shortly thereafter. Then it flattened out for a couple of years, and under normal circumstances it probably would have started to climb again when the economy began to recover. Two things stopped it: Dodd-Frank and Basel III, both of which mandated higher capital requirements and thus lower overall leverage levels. This has reduced Wall Street profits but made the banking system safer for everyone.
In other words: financial regulation FTW. Nothing is perfect, and Wall Street is doing everything it can to undermine Dodd-Frank during the rulemaking process, but if it accomplishes nothing except encouraging less leverage it will have done its most important job.