You base everything on rules, which some Shinigami write, with those rules both being incomplete in terms of the Shinigami's knowledge (they purposefully hold back) and the incomplete knowledge of the Shinigami (Ryuk isn't sure about several things involving the Death Note, is surprised by some things such as being able to use a page that was torn out, etc.).

For all we know, Saitama has reached a level where he would probe yet another exception to the "rule" - although there's actually no rules, only a "how to use" guide, and the Shinigami will have written that in intending the user to write the name of "an ordinary mortal man".

Tl;dr The Death Note has so many exceptions that I don't buy the assertion it'd work when nothing else seems to work.

The way I see it is Saitama has to eat. He feels hungry, and that makes him human enough to be able to be killed by the Death Note.

Saitama was punched so hard he went crashing into the moon, stood up, then jumped back to Earth; not sure we can assume he is mortal, given he should have been killed on at least three counts in that instance alone.

He's cleary not an ordinary mortal man, but we can't really say he is a god, and like I said, anything less than a god can die by the death note

You base everything on rules, which some Shinigami write, with those rules both being incomplete in terms of the Shinigami's knowledge (they purposefully hold back) and the incomplete knowledge of the Shinigami (Ryuk isn't sure about several things involving the Death Note, is surprised by some things such as being able to use a page that was torn out, etc.).

For all we know, Saitama has reached a level where he would probe yet another exception to the "rule" - although there's actually no rules, only a "how to use" guide, and the Shinigami will have written that in intending the user to write the name of "an ordinary mortal man".

Tl;dr The Death Note has so many exceptions that I don't buy the assertion it'd work when nothing else seems to work.

The way I see it is Saitama has to eat. He feels hungry, and that makes him human enough to be able to be killed by the Death Note.

Well, Mumen Rider has accepted his limits. He stays in C-class, and his whole speech to the Deep Sea King is about how he knows he won't win but he has to fight anyways.

He knows he can't win because he just can't, but if the theory was true, he would have been stronger than Saitama and he would have won because he knew he could. The theory was that what you wish to achieve, you can, and Mumen rider wishes to be that strong, but he just isn't.