They're getting on a bit now. John Dean, for instance – he of the "cancer on the presidency" – exudes a gravitas in his early seventies that he never possessed when he was the bright-eyed, impossibly preppy 33-year-old serving as Richard Nixon's White House counsel. Or take Egil "Bud" Krogh, once head of Nixon's notorious "Plumber's Unit", now an avuncular lawyer and lecturer on presidential politics.

Their old adversaries, the Washington Post reporters Bob Woodward and Carl Bernstein, were even younger, 29 and 28 respectively, when the story came along that would change everything. In some ways, they've changed less. Trim and disciplined, measured of speech, Woodward is part of the establishment. Back then, Bernstein always looked the rebel of the pair and still does, fleshy and faintly raffish, bearing visible evidence of a hard-lived life.

And all of them were reunited last Monday evening at an event in the office building that was the scene of the crime that ignited the most celebrated political scandal of the 20th century. In case you'd forgotten, exactly 40 years ago today, in the early hours of 17 June 1972, a private security guard called Frank Wills noticed tape covering the latches of several doors, allowing them to close but remain unlocked. He removed the tape, only to come back an hour later to find them sealed over again. At that point Wills called the police, who arrested five burglars, all dressed in suits and ties but wearing rubber gloves. The building was called Watergate. The rest is American history.

What a White House spokesman then contemptuously dismissed as a "third-rate burglary" would be revealed as the tip of an iceberg of wrongdoing that brought down a president. The affair turned journalists into heroes, and the suffix "-gate" into a gift for lazy headline writers trying to jazz up some minor controversy or indiscretion. Above all, Watergate proved that the American system, when faced with an ultimate test, worked. It showed that no one – not even the man in the Oval Office – was above the law. The country kicked out its leader according to the rules of the constitution and replaced him with another, without even an extra policeman on the streets. But would the system work today? Could a 21st-century Watergate be exposed?

There are some who believe that, had the internet been around, reporting Watergate would have been a doddle. Oh yes? In Watergate, nothing had been put on public record, there was nothing to Google. The internet might have helped on the margins – for instance, for finding phone numbers more easily – Bernstein argues. But, in the end, there was no substitute for old-fashioned reporting, working the phones, knocking on doors unannounced, and following the money.

Also among the speakers were Fred Thompson, who in 1973 was minority counsel for the Senate Watergate committee, and Richard Ben-Veniste, an attorney in the Watergate special prosecutor's office. Between them, they set out six essential conditions to get to the bottom of a scandal of such dimensions: an aggressive press, a strategically placed inside source, a top White House official ready to co-operate with investigators, a tough judge, tapes of key conversations, and a Congress ready to put duty to country ahead of duty to party. Today, a question mark, at minimum, must be set against almost every one.

Probably, a new scandal would throw up another Dean, ready to switch sides and to spill the beans. One hopes that there would be a judge as implacable as John Sirica, a source as authoritative as the then deputy FBI director, Mark Felt, aka "Deep Throat", reporters as resourceful and diligent as Bernstein and Woodward, and an editor as brave and committed to the story as the Post's Ben Bradlee. But would a modern Bradlee have the resources? Newspapers are in financial straits; investigative journalism, requiring time and money with no guarantee of ultimate reward, is a luxury ever fewer media outlets can afford. Independent websites and public interest groups can fill some of the gaps, but not all of them.

As for Congressional bipartisanship, forget it. The cross-party backing that gave the Senate's Watergate committee real clout is all but inconceivable in today's divided Washington.

And presidents, too, are much warier. Modern emails are a treasure trove for historians but tell only part of the story. And, as far as one can tell, there are no longer taping devices that preserve every presidential word, offering incontrovertible evidence to cut short the "he said, she said" disputes with which many scandals peter out.

Without the tapes, Nixon would almost certainly have got away with it. But sitting in Monday's audience was the 86-year-old Alexander Butterfield, one-time assistant to the president. It was Butterfield who, on 16 July 1973, appeared before the Senate committee and, in answer to a question from Thompson, confirmed the existence of a White House taping system. In doing so, he sealed Nixon's fate.

Most depressing perhaps, the campaign finance reforms passed by a post-Watergate Congress are today all but gone, given a helpful kick on the way by the 2010 Supreme Court ruling that reopened the floodgates to unlimited individual campaign donations. Once again, perhaps inevitably, money rules. And even the 1960s building itself looks past its sell-by date. Half the offices are empty (the new owners promise a total refurbishment). Not just the scandal but also the scandal's very setting are relics from a vanished age.