But it does. Population very much does matter. For a war-time economy? You need as many people in the factories as possible.
That assumes warfare in Mass Effect is comparable to present day warfare, furthermore, robots greatly enhance the productivity of labour and even nowadays most first world factories are heavily automized. Population numbers are only important if you make it important.
The Asari are repeatedly called the most technologically advanced. The Salarians are clearly more advanced than the Alliance, and the Turians were who enabled the Normandy to be built.
Technology in Mass Effect is incredibily standardized, and it's unlikely that one Citadel race would gain a superiority of the other strictly through technolgical means. especially when we don't even what these supposedly more advanced technologies even are.
Sure, any outcome is strictly hypothetical. Like who would win a war between the United States and Cuba. Strictly hypothetical. But it's fairly obvious who'd win. The Turians are more competent militarily, have five times the dreadnoughts than the Alliance has at its peak, and has at least five times more fleets than the Alliance, with 32 to the Alliance's peak 6. And it's very reasonable to believe each Turian fleet is larger than any given Alliance fleet, given that the Turians, are well, the Turians.
There's no closing evidence to suggest to the Turians are any more competent at warfare than the Alliance. Whilst the Turians are militarized and have more ships, there is no specific data available as to how the various naval forces would fare against each other in a military engagment.
Let's say you're right. Do you think the Asari, Turians, and Salarians are just going to let the Alliance set any sort of policy regarding Council space without them having a say? Do you think the Volus, as a client race of the Turians, would be happy about it either?
No. What would happen, if the Alliance were to actually try to make a power play to control the Citadel, would be the complete disregard of any rulings the new "council" would set down, followed by the complete and utter annihilation of the Alliance military, and the dissolving of the Alliance ruling body.
More supposition and conjecture on your part. You're forgetting we're talking about a scenario in which the council forces are heavily depleted. Udina literally states that the citadel fleets (plural) were decimated by sovereign. Like it or not, In the event the council should perish the narrative is quite clear about it's implications. Mass Effect 2 even confirmed this by stating "Humanity seized political control of the galaxy".