From what I understand, OpenAI just bought out a significant proportion of the capacity of Samsung and Hynix, which is the big reason prices just spiked. They're two of the three DRAM manufacturers, Micron being the third.
That gives us a good idea as to what Micron is doing here: They have contracts with their other customers to supply DRAM at whatever price they previously negotiated, but now prices are higher, and they don't have to honor supply contracts with Crucial because they own it. So they're taking all the supply that would have gone to Crucial and selling it to the high bidder instead.
Spinning off the brand in that context doesn't work, because then "Crucial" would need to come with supply contracts or they'd have no access to supply and have nothing to sell. Moreover, the supply constraint isn't likely to be permanent, and whenever prices come back down then Micron would still own the brand and could start selling under it again, which they couldn't if they sold it.