The leaders may be motivated by retribution, but I'm sure none of leaders or researchers really want to be a division of MSFT rather than a cool start-up. Many developers may chose to stay in SF and create their own startups, or join others. Signing the letter isn't a commitment to go to MSFT - just a way to pressure for a return to status quo they were happy with.
Not everyone is going to stay with OpenAI or move to MSFT - some developers will move elsewhere and the knowledge of OpenAI's secret sauce will spread.