I would disagree with the idea that it was the Marian reforms that caused the downfall of the Republic.
For one thing the Marian reforms never actually happened. (Or at least they weren't anything done by Marius.) Professor Bret Devereaux has a good article explaining this and showing that the only one of the Marian reforms that might have actually been done by Marius himself was the standardization of the eagle standard for the legions.
This week we’re going to take a bit of a detour because the previous post on the Roman conscription system, the dilectus, sparked some discussion both here and on social media which made me r…
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In particular the army becoming a professional force made up of volunteers was a development that occurred during the reign of Augustus. Marius did use poor volunteers for one of his campaigns but that was in response to a specific political conflict between him and the Senate (the Senate was upset that Marius had used the Popular Assembly to take the command of the war against Jugurtha away from Quintus Caecilius Metellus and thus refused to allow Marius to hold a levy which led to him taking on volunteers), but there's no evidence that Marius continued to rely on volunteer troops after that campaign, and the historical record does show the Romans continuing to use their traditional levy all the way to the end of the Republic.
At any rate the Republic's ultimate defeat was not because of a lack of loyal troops. Caesar had to defeat four separate major Republican armies (and after his death the Liberators that were fighting in the Republic's name raised yet another huge army), so the Republic never lacked for troops willing to fight for it.
I don't think that would help. What brought down the Republic was allowing Caesar to serve as proconsul in Gaul for so long and not keeping the size of his army in check. This allowed Caesar to build up a huge force of very experienced troops that were personally loyal to him. And with the wealth he obtained from his successful campaigns in Gaul, he was able not just to cement the loyalty of his troops (Caesar doubled the pay of his men right before the civil war), but also to fund a lavish building program in Rome (that won him a lot of popular support in the city) and buy the support of key politicians.
And how did Caesar obtain that long command? Through the popular assemblies. (He got his initial 5 year command through the Lex Vatinia passed in the Tribal Assembly during his consulship in 59 BC, and then got a 5 year extension enacted in 55 BC through the Lex Trebonia which was also passed in the Tribal Assembly.) Likewise, Caesar used friendly tribunes of the plebs (who are elected by the Tribal Assembly) to block the many attempts by the Optimates to terminate his command early.
The kind of reforms you are talking about would strengthen the popular assemblies and weaken the Senate. Thus they would only serve to make it easier for men like Caesar to get and hold extraordinary commands, since it was in the popular assemblies that granted such commands. (Caesar was not the only ambitious individual to obtain an extraordinary provincial command through the popular assembly after all. Prior to Caesar, Marius used the popular assembly to get command of the Jugurthine War, and Pompey used the assembly to get his commands against first the pirates and then against Mithridates, and after Caesar's first proconsulship, Aulus Gabinius, Lucius Calpurnius Piso, Crassus, and Pompey all used the popular assembly to get extraordinary provincial commands for themselves.)