According to me, even if Bhishma couldn't be killed but still there were other ways of stopping him. If you know about the story of Samudra Manthan and what happened afterwards with Rahu, you can probably imagine what could Lord Krishna might have done. As per the Adi Parva of the Mahabharata the following describes what happened with Rahu after he drank Amrit:
And all the gods at that time of great fright drank the Amrita with
delight, receiving it from Vishnu. And while the gods were partaking
of it, after which they had so much hankered, a Danava named Rahu was
also drinking it among them in the guise of a god. And when the Amrita
had reached Rahu's throat only, Surya and Soma (recognised him and)
intimated the fact to the gods. And Narayana instantly cut off with
his discus the well-adorned head of the Danava who was drinking the
Amrita without permission. And the huge head of the Danava, cut off by
the discus and resembling a mountain peak, then rose up to the sky and
began to utter dreadful cries. And the Danava's headless trunk,
falling upon the ground and rolling thereon, made the Earth tremble
with her mountains, forests and islands. And from that time there is a
long-standing quarrel between Rahu's head and Surya and Soma.
So as we can see, Rahu had consumed Amrit so he could have been a problem as he became immortal but still Lord Vishnu cut him off, disabling him to create any major troubles. Lord Krishna had used Wheel in the form of Charka, so i believe he might be trying to do the same here. If Bhishma's head would be cut off then he won't be able to fight but can still be alive.
We have another example of the same act with Barbarik (son of Ghatotkach), who could have proven to be a problem for both Pandav and Kaurav army, so Lord Krishna also used the same technique with him, i.e., he discussed with Barbarik and it was mutually agreed that Barbarik's head will be cut off so that he couldn't participate in the war.