And non competes can be as airtight as you like.. they are completely unenforceable in California which is where OpenAI's HQ is based and where Sam Altman lives.
First the loyal will jump ship. Followed quickly by the mercenaries who see the chance to join a new rocket ship from the ground up. Then as OpenAI's shares tank on the secondary market the rest will decide they've seen enough of their paper money burn and cash out.
OpenAI will survive but its going to be a much smaller company and a much smaller valuation.
As for Microsoft I'm guessing one of the strings Nadella was pulling was threatening to revoke credits, resources and even use by his teams and I'm sure he would be interested in investing in whatever Altman starts next and dedicating those now spare machines to the new enterprise.
California's non-compete laws don't cover "trade secrets", only your ability to use your expertise to pursue employment in your chosen field. In other words, if you're just a regular programmer, you can go work wherever you like or start a competing company. If you're a principal scientist or architect, you would be in danger of violating your contract. Anyone who went with Altman would presumably have deep inside knowledge of OpenAI's secret sauce and therefore be restricted.
But there is an alternative scenario.. in order for Microsoft to avoid loosing any momentum they might offer Altman an insane amount of money to become the John Giannandrea of Microsoft and bring as many of his recent colleagues with him. And for Altman this might be the easiest way to not loose ground as well with Microsofts patents and license agreements.