It sounds so good in theory, but in practice: 1. Frequently old code needs to be touched or refactored. Feature flag would not be enough. 2. Even feature flag

It sounds so good in theory, but in practice: 1. Frequently old code needs to be touched or refactored. Feature flag would not be enough. 2. Even feature flag itself can be a risky addition, and might affect existing customer usage. Most of the time old code does need to be touched, there really aren't those perfect new isolated features, at least in my experience.