It's as unanswerable a question as "how many lives would have been lost by dangerous medications with long-term side effects that didn't become apparent in the term of testing?"

We can't really know whether it was a net good or a net bad because we can't take a look at the reality that didn't happen. What we do know is that there were deadly medications on the market before the FDA. There were medications available over the counter that were outright poisonous. Arsenic and mercury were in common medicines.

It is a balance. You can sacrifice lives through inaction, or sacrifice lives in the way of progress. Lack of regulation is bad, but so is overregulation.