It wouldn't be the first time Tevinter underwent radical changes. While a Circle system that copied Orlais would be ideal, the period of Transfiguration, non-mages became the Highest ranking clerics, had seats reserved for them in the senate and slavery was greatly diminished and tevinter was still Tevinter. It was the Altus who survived that led to what Tevinter is today by making the economy nearly entirely reliant upon the slave trade as well as forbidding non-mages from assuming any position of authority. Neither remake a country from scratch, they relied upon existing infrastructure and either added or removed parties.
Ultimately, so long as the notion that a mage is worth more than a mundane exists while mages hold 100% of political power within Tevinter, abuses will be common.
Ideally a democracy should be formed in every nation, since that would give all citizens an equal chance at proving their ability, determining their laws, and generally getting the most out of life... but when analyzing a work of fiction, idealism can only go so far. We're talking about feudal societies that haven't even come close to the concept of civil rights or equality... the only one that even attempts to view all of its citizens as equals is the Qunari, but they are hardly an ideal civilization for a number of reasons...
Aside from Orzammar, every nation in Thedas is ruled by humans, and those humans give preferential treatment to other members of their own race. Every one of those nations has laws in place that put humans above other races, and often strip non-humans of legal standing. The severity of these laws differs from region to region, but they are always present to some extent (aside from Rivain perhaps, but we know little about them)... There is no difference between these practices and what goes on in Tevinter... it's just that in the case of Tevinter, one group of humans is practicing discrimination against another, with a few random elves thrown in on both sides. I'm not really sure why you consider this to be so much worse... I think that systematic oppression is wrong no matter who is practicing it. IMO there is no reason to single out Tevinter when every nation is like this. Again, the fact that mages are the ones in charge does not make these crimes any more severe.