In most places (certainly here in Europe) the alternatives to flat-pack furniture are not cheaper, quite the opposite. Unless we're talking second-hand, which is obviously the most sustainable option.

On quality, I have an apartment rented to students and the Ikea furniture in it is still going strong after years. I make sure to tighten the screws and bolts periodically, but otherwise it's as good as new. I would not have bought "disposable" furniture, no matter how cheap. Conversely, my experience of furniture (and lighting and other fittings) from department stores and hardware stores is that it's three times as expensive as Ikea, three times heavier, and yet obviously poorer quality.

My theory is that if you want quality you are better buying from Ikea. That's simply because any given product will have been bought by millions of other people. By definition, a lot of thought has gone into it and design flaws will likely have been eliminated long ago.