There are many firarms with a free floating firing pin (The M1 garand, M1 carbine, AK47, etc.). Again, it was nothing wrong with the design, it was that people were not cleaning regularly after shooting or they didn't clean it properly after they bought it, so it was filled still with cosmoline.
Something "wrong" with the design, no.
The best possible design? Also no. Especially when you consider that the SKS actually
was designed with the firing pin return spring.
Especially in the context of a U.S. military rifle, I am inclined to argue that our government has no business designing a standard infantry rifle which is not equally useful in the hands of citizens. A lack of firing pin return spring might seem like a tolerable design in a solely military setting, but in a civilian context it is a ludicrously unsafe design. You simply cannot rely on people not to use ammo which is considered interchangeable in one gun when every one else does it with other guns.
In the context of civilian ownership, lack of a firing pin return spring is slightly more of an issue than "oh, well, I didn't clean it, so it jammed." I have several guns which might, occasionally, jam when seriously dirty. I would be very uncomfortable with any gun in my collection which might suddenly go into full-auto because of some grime and a primer which is "too soft".
Actually, I've made the following argument before, and I'm inclined to make it again:
Any firearm design which allows a firing pin to strike an exposed primer, and relies totally on that primer's hardness to avoid discharge, is of negligently unsatisfactory design, is totally inappropriate in a civilian setting, and is therefore not an appropriate weapon for the US government to manufacture as a standard infantry rifle.
I mean, is it really that hard to add a spring? Or, in the case of the SKS, to not remove a spring which was supposed to be there in the first place? The answer is, no, it is not. And yet so many folks rush to defend such a sloppy design oversight, almost as some kind of intentional design feature.
It is foolish to imagine that a rifle will only be used in a military setting (this has literally never been true about any rifles), and I would argue that any designer who makes such an omission is guilty of negligence with regard to the safety of the eventual civilian owners of the machine he has designed.