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.