As history actually turned out, there are two very good reasons why the Gunpowder Plot had to fail. The first was that the plotters were caught in the double-bind of most early modern conspirators: in order to make a rebellion work, it had to involve a lot of people, but the more people who knew about the plot, the more it was likely to leak.
In the case of this one, the means of effecting it, by murdering most of the English political élite, was so sensational and so morally disturbing to most people, that the chances of somebody blowing the whistle on it were unusually high.
That is exactly what happened; one of the people brought into the plot in its later stages (probably the unstable Francis Tresham) told an opportunist peer, Lord Monteagle, who tipped off the government. The other reason why the plot was a guaranteed failure was simply that the powder would not have blown.
When it was moved to the Tower of London magazine after Guy Fawkes was caught, it was discovered to be `decayed'; that is, it had done what gunpowder always did when left to sit for too long, and separated into its component chemical parts, rendering it harmless. If Guy had plunged in the torch with Parliament all ready above him, all that would have happened would have been a damp splutter.
'...the plot was a guaranteed failure was simply that the powder would not have blown.'
Both these fatal weaknesses were contingent, however, on one accident of history; the postponement of Parliament. It had originally been scheduled to meet on 3 October 1605, and only the lingering traces of bubonic plague in London made it seem sensible to put off the occasion for a month.
Let us suppose that this one variable had been removed, and there had been no plague in the capital that summer. Parliament would have met a month before, very probably when the gunpowder (stored in the cellar since July) was still lethally effective, and arguably before one of the less discreet plotters had lost nerve enough to talk to Monteagle.
We are thus in a real position to suppose that in October 1605, King James, Queen Anne, and both Houses of Parliament would indeed have gone sky-high together, leaving the Catholic conspirators ready to seize the kingdom. What would have happened then?