The defence I am giving further up is based on how it appears to a Lavellan who is not in a romance with Solas. He never gets told about the slave markings. In fact if you kill Corypheus quickly enough, you never even get his mocking comments. All he sees is the vallaslin being removed by Fen'Harel. So nothing about nobles marking their slaves. He also never met Felassan and when talking with Briala, just gets told ancient elf society was not much different from modern ones.
There have been instances of people willingly working on a religious monument with great industry without them being slaves. In fact working on cathedrals and abbey buildings was a useful way of supplying employment in the local community and the people put heart and soul into the effort because it meant something to them. When you see the thousands of elves maintaining a magical ritual that builds the Grand Sollanium there is no mention of slavery and it is unlikely that unwilling participants would be able to maintain that amount of concentration in the job at hand. It is also unlikely that thousands of servants/slaves could have built that statue in a single afternoon without magic. The memories of the reaction to his action do not suggest that people were pleased and relieved to be free of their gods; on the contrary they wanted them back.
Now I don't dispute that at the end the Evanuris had become corrupt. May be that is the nature of things. Solas seems to suggest as much. It could also have been because of something external acting upon them, such as red lyrium. That seems to be sufficient to turn anyone into a raging megalomaniac.
What I am trying to get at is how culpable were the entire pantheon in Mythal's death. What led up to it? I don't know how long Solas had been around but apparently there had been thousands of years of elven history when the empire was built, the great magical structures he admired were put into place, magic was used by all people as easily as breathing and he recalls it all with nostalgia. He even admits that the Evanuris weren't always seen as gods and therefore likely they weren't tyrants either. They could well have been the guides and teachers that the Dalish remember but this was before the war that started them on the road to corruption. Presumably the Evanuris became generals in the war because they were considered best able to lead or the only ones willing to do so. So instead of simply saying that the entire Dalish belief system is invalidated by what the gods became, it might be better to question why this happened?
Just look at Andrastrianism. It was started, so we are told, by someone who wanted to free her people from tyrants and slavery. Then the faith was first adopted by the chief of those tyrants, in a blood bath of his rivals, and slavery exists to this day under the banner of the Maker. Down south, another tyrant, Drakon, achieved hero status because of his efforts in the 2nd Blight but he also wiped out numerous other religions and rival cults to the Maker, until only his own remained. Yet Andrastrianim is celebrated as a great unifier. Like hell it is. It is a convenience that keeps the corrupt rulers in power.
Before tearing down the modern world, perhaps Solas should think more carefully as to why things go wrong. Still if I could get rid of all the corrupt systems of government in Thedas in one fell swoop, without the ordinary people suffering as a result, I'd sort of go for it. What a minute, isn't that what the Qun were going to do? Of course, naturally I didn't want them to succeed.
Still the actual system of governance currently promoted by the Dalish is actually a pretty good basis for a bigger community. Just remove the requirement for the Keeper/Guide to always be a mage. They believe this is how their ancient world was governed and for all we know, perhaps early on that was the case. That's why simply rubbishing their belief isn't good enough, particularly when we are constantly asked to uphold political systems that seem no different to that which Solas condemns. You do actually need laws that people abide by and someone to lead, just not the ones we currently have. If any of the rulers in Thedas actually upheld the moral code of the Maker, I wouldn't have such a problem with Andrastrianism but as it stands, it is a dead faith: their god doesn't respond to them and they don't live their lives by his teaching.
Solas also tends to change his motives in the narrative. First he raised the Veil because he wanted to punish the Evanuris, then he says if he hadn't done so, the Evanuris would have destroyed the world. So he destroyed the world of the elves, to prevent the destruction of the whole world but was upset because in stopping the latter, he didn't anticipate the former? How were they going to destroy the world? How is putting everything back as before going to change anything? Even if the corrupt Evanuris were destroyed, how can he be sure a new set of super elves wouldn't arise to rule over the rest once more?
I also get this nasty suspicion that the way the narrative is going, we are going to discover the Evanuris were really responsible for the Blight, thus leading to yet more suffering for the elves as people blame them for that and ultimately a lot of dead elves because they end up on the wrong side from our hero PC. If that is the case, I think I'll just decide not to play.