Veilguard gave us the reason why Solas, a man who seemed so distrustful of despots and tyrants, was yet so skeptical of an Inquisitor who hinted at sharing power.
It's because he had to deal with the oligarchy that was the Evanuris. The idea of 'sharing power' is naive and pointless to him if there is no one at the helm giving the final say because it either devolves into stalemates, or someone with the biggest personality comes to monopolize the power regardless, i.e. Elgar'nan. If there is to be power held by someone, Solas would prefer a single ruler who could lead with wisdom and prudence, who is able to listen and use their judgement to filter good and bad information (which is what Wisdom does), to preside as a referee or an overseer of sorts to make sure everything is in working order and to ensure everyone is playing nice to step in and correct only when it is needed.
This is why when he asks you what you will do in the aftermath of What Pride Had Wrought, he disapproves when you say "I will rely on our allies". Assuming he has faith in your abilities to lead, he disagrees with this sentiment. You have shown yourself capable of handling the power that has been given to you. To divvy up your power out of some reflexively good-natured desire for the sake of democracy would prove disastrous. The more cooks in the kitchen, the more different agendas and personalities that need to be juggled, the more discordant that the operation becomes because it is far easier for power to be effective when you have a single accountable, capable person who has the final say.
Not only that, but given the allies that the Inquisitor does pick up, it's a nice gesture that would be squandered on people who didn't have enough sense to put aside their differences and work together without being strong-armed or getting their asses saved. It's a tiny drop of ubermensch, "great man theory" tempered with "let's be absolutely real here. Why would you ruin a good thing you have going? Why switch up what's working? To avoid hurting peoples' feelings? To make them feel included? You're a good friend/ma'vhenan but please don't be stupid about this."
And then you have this banter between Solas and Vivienne
Solas is skirting around the fact he is an ancient elf (though of course he is not omniscient) but in his ideal state of the world he would be omniscient + immortal and would be the arbiter to punish wrong doers. He would not allow vigilantes this power because in this thought experiment he has the most sound judgment. Wisdom is dipping a bit into Pride here, Pride and the mind of a man who believes he has the wherewithal to judge a person fairly.
But then, ho ho, you get THIS banter with them
If there is anything Solas hates, it's naked ambition and overestimation of one's abilities (yes this is hypocritical). To Solas, Vivienne is undeserving of this political power she is casually dropping her interest in pursuing because from his perspective she has done nothing substantial to help mages. For Solas, helping mages would be securing their freedom and place in general society. For Vivienne, it's keeping them sequestered so they aren't hunted down by superstitious magic-fearing Andrastians and can be kept monitored for possible abominations. Solas sees Vivienne as someone who only wants power to play her little political games in court and become someone who is eminent and powerful for herself chiefly. If mages benefit, it is coincidental or an afterthought, or so he predicts for her.
He prefers blatant and honest declarations of activism, because then someone can be measured by their actions thereafter and graded based on how they've lived up to them. Vivienne going "Well others have failed, I could try my hand at it" pisses Solas off because it comes off as so flippant to him. He takes conversations about power and politics very seriously, as you can tell, and with his past it's understandable given how he had to fight millennia-long rebellion dealing with the Evanuris and then walked through how many dreams and memories of how many kingdoms and civilizations and villages and towns and cities crumbling because of power struggles and ill-fitted rulers seeking only the position while eschewing the responsibilities it came with.
