The St Brice’s Day massacre certainly didn’t have a significant effect, since after that massacre the Danes conquered the Saxon part of England – twice! Once under Sweyn Forkbeard and once under his son Cnut the Great. The ‘Kingdom of England’ under Æthelred didn’t include the Danish parts of England.
Both times the Saxons regained control, mainly due to Danish internal conflicts. Harthacnut (the son of Cnut), a subsequent Danish king of England, was apparently content to leave the whole of England to his Saxon half-brother Edward the Confessor on his death.
The Danelaw – parts of England ruled according to Danish rather than Saxon law, whoever was in charge – only ended with the Norman invasion.
It’s unlikely any one incident had a significant effect, which also is not what I claimed. Lots of incidents over long periods of time can change nations, though.
It was never close to a 20th century liberal democracy, and was (as with plenty of 20th century liberal democracies) heavily dominated by a rich elite, who had power far beyond their numbers.
But it absolutely was a representative system of government, and the biggest that had existed up to that point, and would exist for a millennia or so afterwards. The people’s assemblies had genuine legislative power, and were genuinely representative bodies that all free male Romans were eligible to take part in them. At the height of the republic, after the social wars, that meant 100,000s of people, far more than any other equivalent representative that would exist for centuries (admittedly you could argue that it was too large for ancient society to support and was one of the reasons for the collapse of the republic).
He ultimately sided with them to oppose Ceasar he also did plenty of dallying with the populares too (he was one of the triumvas with Ceasar and Crassus, both solidly populares.) Also its anachronistic to think of populares and optimates as modern political parties with a raft of opposing polices, they are just labels applied to the sides of the conflict.
And he absolutely supported several reforms, his reason for joining the Triumvirate was ensure reform land was carried out (which ensured the loyalty of his veterans as a convenient side effect, but it was definitely a reform). He also spearheaded other reforms to the political system including restoring the powers of the tribune of the plebs. He was no champion of democracy and liberty, and maybe he’d have set himself up as autocrat if he’d defeated Caesar, but Pompey and plenty of others saw the flaws in the republic but did not see it as dead in the water. They were actively trying to reform it so it could survive.
It was definitely corrupt and it (or the massive inequality associated with it) was definitely unpopular in some quarters. But that doesn’t mean its collapse was inevitable and hence it was “good thing” it was toppled by a skilled ruler (or pair of rulers in Caesar and then Augustus). Caesar deciding to start a civil war and collapse the roman republic made the collapse inevitable.