Toyota quality has not declined, the Corolla is not worse than it was in the 90's, and the frame and the suspension do not sag. The Corolla still scores a 8 or 9 out of 10 on most surveys, and it is currently the best-selling car in the world.
I have tons and tons of friends who aren't thinking of selling their Corollas, because they have had nearly perfect ownership experiences with them, which isn't surprising given that Toyota makes some of the best cars on the road.
While working with a fellow mechanic on a Corolla this weekend, we both were joking about how poorly (and cheaply) these cars are made. The unibody is rather weak, and yes, sagging does occur on these cars. The engines and transmissions are fairly solid and reliable, but the rest of the car is pretty cheaply made.
Most magazines are in agreement. Every test or comparison I have read ranks the Corolla at or near the bottom of the barrel. Basing current quality expectations on 14-year-old cars is not realistic. All car makers vary in quality a great deal over that length of time. Just look at Hyundai and Kia. In '98 Kia was regarded as cheap, disposable transportation. Now Kia and Hyundai are increasing in sales while other brands (Toyota included) are dropping. The Hyundai Elantra has finished 1st in the last two comparison tests I have read, beating out even the Civic, Focus and Chevy Cruze.
I have yet to see a single car - other than a few that were old and had spent years in snow belt states - have serious frame issues or "sagging issues". Anyone that knows anything about structural engineering will tell you that a unibody construction is by far stronger than a body-on-frame vehicle. I can attest to this because one of my other vehicles is a '55 Mercury. Sure - it's got a truck frame. But the sheet metal just hangs off of it and provides zero structural integrity. In an accident the '55 will crumple onto itself.
Not sure where this "sagging frame" story came from regarding Corollas. A lot of my friends own these cars, and not a single one has had this issue much less any other serious issue.
These are well-made cars, and they rank highly on just about every report I've read. I believe I'll stick with a brand that actually has a proven track record versus a brand that has a lackluster one.