Tevinter is already constantly sustaining losses to Par Vollen. They don't have a limitless population. They encourage a structured nobility that encourage mage-heavy lines to keep breeding and producing as many mages as possible. It's not realistic to believe they're constantly losing huge numbers to abominations and not losing the war, let alone having revolts. The conclusion just doesn't make sense. They have a finite number of people. They already kill the commoners constantly THEMSELVES, for blood sacrifice. How many people do you think they can lose and have anyone left?
Uh, lots. And for quite some time.
As long as the population growth rate is higher than the population death rate, the situation can mathematically continue forever- the number of people they lose is irrelevant without a context to the number of people they gain. This is especially true with the fact that slavery is a means for artificial population increase.
And, of course, there is no evidence that Tevinter is on a sustainable path in the first place. The only relevant data we have is that it has not collapsed yet, not that it is demographically stable.
Your argument hinges on a number of assumptions that no one is claiming or supporting- starting with that the Qunari war is a major population drain for them (extremely unlikely- even far more efficient total war systems of the industrial era didn't do that), that the Abominations are a statistically rather than socially significant source of deaths, that revolts are a statistically significant population cost, and so on. You're also casting costs in a false delimma of either being catastrophic or irrelevant socially ('constant hemorrhaging'), while ignoring the factor of time (treating the costs as cumulative to a fixed population, rather than distributed over time).
The Tevinter can have all of these and still maintain positive population growth (or sustainment, or even non-catastrophic decline) quite easily- they just don't have to abide by your inflated fictional attrition rates.