Satya planned this all along
Sam Altman and his team are now part of Microsoft.

I can see OpenAI becoming less and less relevant over time, investors are about to lose a lot of money over this debacle. Satya pulled a 4D chess move before the markets opened.
OpenAI is just the 'AI Safety' arm of Microsoft when you boil it down. All the cautious folks are in one place to manage at a distance while Sam gets to play God internally as CEO of this new team. As new CEO of the Advanced AI Research team does that mean he would also have oversight and input into the OpenAI roadmap? That would be hilarious if true. I mean, no-one really is taking much notice of ̶E̶d̶ ̶S̶h̶e̶e̶r̶a̶n̶ Emmett Shear taking over as Interim CEO.

Unfortunately, we're back to the same old Microsoft v Google v Apple power play again.
But maybe Sam and Satya were already in discussions for the new AI Research team and the Board only just became aware 11th hour. Sam could have been thinking he would run both at the same time perhaps, Board saw too much conflict then dumped him. Satya leads the negotiations because he has to make sure there's an outcome that protects everyone's interests.
Mira wanted Sam to stay, it was obvious that if he left then there'd pretty much be an exodus from OpenAI leaving it floundering. As Interim CEO of the day it makes sense, placate the Board and have Sam stay in some capacity whilst setting up the new venture in Microsoft. But Ilya's ego is too big and the Board refuse to budge setting off this chain of events all over again. This time Sams walks off arm in arm with Satya, and Sayta pulls off the greatest acquihire in tech history without spending a dime.

There was never a rehire option on the table for Sam at OpenAI, the discussions were to lay out the plan going forward but had to be accelerated as a result of Board ineptitude. This could explain how Satya could mobilise the resolution so quickly and get it approved himself, there had to have been prior discussions.
Literally watching OpenAI disappear in real-time"OpenAI is nothing without its people." has become a rallying cry across Twitter. It's a signal to Sam and Greg who is ready to jump ship to Microsoft.
Notice that Satya says "Microsoft stays committed to OpenAI" meaning "we'll continue to provide them with Azure services until they're no longer useful to us." which might be sooner than expected if there's no employees to maintain ChatGPT and DALL-E services and APIs.
Given the amount of staff announcing their intended departure I don't expect OpenAI to last or have the same industry impact, nor be of interest to investors in its current shape. I mean, who really wants to continue to invest in a company manned by AI safety researchers who don't like innovating?
Microsoft's investment was mostly Azure cloud credits so that can be pulled at any point and they could absorb the write off easily.

It would be funny if OpenAI had to turn to Microsoft and the new Advanced AI Research team for help...
Question now is: how many users and businesses are reconsidering their subscriptions? How many consultancies and services based on ChatGPT capabilities are now rethinking and scrambling around for second options?
The fallout from this is far from over.