It’s true that a new change can intriduce more bugs, but I have a suspicion that the excel team is working with brains and a test harness that most software projects don’t have the time or money for.

The problem is: if you were the chief product manager of Excel you would risk yourself? New tests are required for new features and it's a difficult execution that money doesn't buy by itself. Is this a significant change to risk your job?

Big companies have all the money to modernize their systems yet they know the execution is difficult if not impossible to achieve in time. The classical story about running an older operating system and/or application that works instead on venturing on modernizing it.