Hi Mightyman,
Yes … 3" out of the top 3 3/4" in the mattress is a lower density polyfoam which is a much lower quality material and 3" is more than the guidelines of “about an inch or so” that I would suggest (see here). All the old Sealy Springfree latex mattresses (and the very similar Stearns & Foster luxury latex mattresses which replaced them) have a similar weak link and use more polyfoam in the comfort layers than I would be comfortable considering.
The latex will probably still be in good condition and some people have even cut open their sagging mattress and done “mattress surgery” to replace the polyfoam with a higher quality material (see post #2 here)
Phoenix