I have done alot of research on this subject and still find myself a bit confused, but here is what I know. If you want a sofa that will last you need one with a solid hardwood frame and 8 way tied springs. This level of quality is gonna cost you, though...at least $4,000 for a sectional. Less for just a sofa. I would be surprised if Costco carries anything of this quality.

$4,000 for a sectional is like admitting you think you have extra money and should pay more taxes!

texasdad, did the frame on both the original and replacement couch go sagging in the same place?

Don't be surprised about the pressboard construction now. Folks build new houses with wafer board floors these days.

You're on the right track flipping over sofas and looking inside. Also beware of super long sofas or sectionals with high ground clearance and no center legs. If the "superstructure" doesn't have trusses like a bridge I bet it will fail.

does Costco still offer warranty for that couch? I heard Costco has life time satisfactory warranty

The couch replaced under warranty and the current one have frame damage in the exact same spot.

Costco does offer the lifetime satisfaction policy, but I'm not sure how that would work with a several year old couch? I guess it would be returnable?

You won't get good leather, a hard wood frame and 8 way at Cosco, nor will you find said couch for much under the $2500 price point, and I will venture even to say under the $3K mark.
Cosco will have goods priced for the bulk of their customers, which do not include higher end furniture.