Strictly speaking, the S-series planned for 1962 was not the last DeSoto. Chrysler management briefly considered a 1962 DeSoto cloned from the definned Chrysler Newport. Engineering released an ornamentation drawing of it, the only difference between it and the Newport involving grille emblems.
It's doubtful that Chrysler seriously intended to sell this DeSoto. Indeed, the drawing may have been done only so that officials could honestly tell the press, which had been asking pointed questions about DeSoto's future since 1959, that Chrysler really was working on new models.
On that score, one car magazine reported in early 1960 that, in light of falling sales, DeSoto would offer only a compact for 1962. Presumably, this would have been a Valiant-based car instead of an all-new design, though the report (actually a bit of gossip) didn't comment on that. Perhaps they got their rumors mixed up with insider tips on Dodge's Valiant-clone 1961 Lancer.
As for the stillborn S-series DeSoto, Fred Reynolds remembered inspecting the completed metal prototype six months after the program was killed. He recalled it looking grotesque, awkward, already dated.
All told, he was glad the company decided not to build it, which means he couldn't have much liked the aborted Chrysler or Imperial either. Given the poor showing of the 1962 Plymouth and Dodge, it's probably just as well the S-series was axed.
The upshot to this story is that DeSoto died in name only. Less than a year after its burial, the make was effectively resurrected at Dodge in the full-size Custom 880. Hastily fielded to offset underwhelming sales of those downsized 1962 "standards," it arrived with a pair of hardtops, like the 1961 DeSoto line, but also a hardtop wagon, four-door sedan, even a convertible.
All combined a 1961 big-Dodge front with a 1962 Newport rear, which meant the reborn big Dodge was essentially the same car as the 1960-1961 DeSoto, Chrysler and, yes, full-size Dodge. The two Custom 880 hardtops even cost about the same as the last DeSotos, and the entire line sold well, doubtless due to its conventional, inoffensive styling.
The Custom 880 continued earning good money for Dodge right on through 1966. Given that, one suspects that DeSoto's rapid decline, like Edsel's, stemmed less from a changed market than a "loser" image and increasing rivalry from sister divisions.