That would be quite a gamble. The unexpected can easily happen in politics. There's no guarantee he will succeed. It's particularly unlikely the Dems will sink below 40 votes in the Senate, and thus lose the power to filibuster. And, in the meantime, four years will have been wasted.
Whereas going after the Clintons from the start could very well have so discredited the Dems that they would do as badly as the Republicans did in the elections after Watergate.