I love the what ifs these things throw up. What if it had worked? It would have changed the course of history so much.
Or not ?
The whole plot was based on the unicorn based idea that the moment the nasty King was dead, the oppressed masses would rejoice and welcome a Catholic monarch with rose petals and blowjobs.
I suspect in reality in the moments after the explosion, there would be a pretty rapid reassessment of which side of the divide you were on, and more than likely - religious differences included - people would have rejected the plotters and their engineered outcome.
And that's before you factor in the fact that the other European powers would have to revise their strategies - a Catholic English monarch would have made continental alliances very messy.
In fact, I really wouldn't be surprised if the Monteagle letter was the result of a foreign power (Spain, France) who whilst publicly talked up the anti-English-protestant country, in private were jostling to try and engage England against the other. France did quite well out of the Anglo-Spanish dust-ups. And having a protestant England across the channel from France probably suited Spain, as it tied up armies preventing invasion.
International intrigue - a game the whole world can play. And has. Frequently.