I agree with you that Shadow shouldn't be banned, but I don't think all your points are very convincing. Death's Shadow isn't the type of card that can be easily slotted into any deck, it's a build around. It's never going to be like Burn splashing blue for Treasure Cruise, or black for DRS. The power of the card is evident in the fact that every single 3 color combination that includes black has been tried as a Death's Shadow shell, with Grixis being the most successful so far. That said, that's not a reason to ban it, because it's not really usurping other archetypes (except for maybe classic Jund), it's created new ones.

On the talk of unbannings, they cannot unban Preordain anymore because Grixis Shadow would play it. That one's completely off the table.

SFM, on the other hand, is reasonable. The most likely decks that would run it would be UWx Control, Abzan midrange, and Death & Taxes, none of which are tier 1 strategies right now. The best of them is Abzan in tier 2. There's actually an important benefit to unbanning SFM. First of all, it would take attention away from Death's Shadow. Some people playing Shadow would probably switch to a SFM deck because she's a powerful card and people enjoy playing with powerful cards. Secondly, although SFM herself isn't great against Grixis Shadow and its Kolaghan's Commands, the decks she goes in are some of the worst matchups for Grixis Shadow. If Abzan Midrange, UWx Control, and Death & Taxes all become bigger players in the metagame, they would serve as an effective check on Death's Shadow ever going too crazy.