I've spent today watching All the President's Men and also listening to the commentary by Robert Redford, who produced as well as acted in it.
Redford wanted to make the film even before Nixon's resignation. He was travelling with a group of political journalists at the time of the Watergate break-in, and some of them explained to him that, although the published stories were about crazy Cubans, they all suspected there was more to it and that it pointed to the GOP. They also told him the real story would never come out, because of the interests aligned against this.
It underlines the "slow burn" theme cozie has frequently mentioned here: it took a long time for the investigations by the reporters to get anywhere significant, they were sniped at for each story, and it took even longer for the conspirators to be prosecuted and for the story to seriously impact Nixon.