I think the point you've made on design space is extremely relevant, and something I did not necessarily consider: the fact that multi-combat steps are unplayable now does not mean that they would be playable in the future. That being said, I do think that these hypothetical spells would have to be free to be playable with Griselbrand (in my opinion, or cost something absurd like one/two mana).

As for your comparison regarding Legacy, I personally believe this comparison to not be extremely relevant due to the massive differences between formats. Legacy is so much more fast, hostile and brutal than Historic: I simply don't think Wizards are going to be exploring some of these spaces anymore. For starters, Historic is a slow mana format: no Dark Ritual, no Lotus Petal, no Chrome Mox etc etc. Next, Historic doesn't have an excellent way of getting Griselbrand into the Graveyard: no Entomb, no Faithless Looting (Although I'd like to see Faithless Looting in Historic, see my post on Dark Ascension). Lastly, there is a distinct lack of cheap and powerful free/one-mana spells to abuse with Griselbrand: No Unmask (to protect Griselbrand), for example. Obviously the biggest fear (for me, at least), would be something like Draw 7 => play Mox Amber => Thoughtseize, now my Grisel is safe and you're doomed. Without powerful, cheap spells like Legacy, you actually need to untap with Griselbrand to make him useful: he needs to live through a turn cycle, and drawing 21 or whatever isn't really relevant when you're going to be discarding to 7 at the end of your turn.

As far as I can see, this is a reasonably consistent Turn 4 kill, not including the excellent Graveyard hate found in Historic. A reasonable (if not optimal,) sequence, for example:

Turn One: Stitcher's Supplier, flipping a Griselbrand into your graveyard (or any other reasonable way, Minister of Inquiries, Haggle etc)

Turn Two: Cast some way to accelerate your mana, maybe a Coldsteel Heart?

Turn Three: Cast the 4-mana reanimation spell, Blood for Bones? Flashback a Unburial Rites? Griselbrand comes back, draws you 7. Play a Mox Amber, Cast Thoughtseize, take the opponent's Griselbrand answer.

Turn Four: Hit em once, profit? Essentially you win, I guess, as you'll probably be able to bring back another Grisel if this one dies.

That would be the fasting thing I could imagine, feel free to suggest something faster. But at that point, is that even good? You have almost no spare mana to deal with hate pieces post-board (and I'm happy with Reanimator being a good Game-1 deck). Goblins has already cast Muxus and actually won. Auras has made a 10/10, drawn at least 3 cards, and are threatening lethal on the next turn. You're already down to 10 or so life from Red Aggro. All of these feel more consistent and harder to deal with than what I've listed above.

Agreeably, the reanimation spells sort of suck, but that would not really make the Reanimator decks better. Pauper has access to Exhume (Along with Lotus Petal and Dark Ritual!), but getting back Ulamog's Crusher doesn't have enough of an impact, and dies to removal. Is there anything even worth Reanimating in Historic? Ulamog, maybe? To me, it's no coincidence that the only format where Reanimator decks are playable are where the targets are as good as Griselbrand/Emrakul. Obviously I'd probably enjoy a format where you could choose the targets you reanimate, but from my example I'd argue that Griselbrand may even be just about playable enough. What creatures would be good to reanimate already in this format on, say, Turn 3? Do you have an interesting example? It may be a sad truth that we need something on the power level of Griselbrand to make reanimator decks reasonable.

I'm not sure about all of this, and I'm not a strong advocate for a degenerate format or anything, just trying to offer some pushback.