If you want to achieve a smooth fade-out of a sound when playing a new instance of a sound, there are already ways to achieve that behavior in FMOD Studio, using combinations of AHDSR modulators, event instruments, trigger conditions, and command instruments.
Stealing is so called because it allows you to “steal” one voice or event instance in order to use it for something new. Gradually fading out one event instance or voice while starting to play another isn’t stealing the old instance, but playing two instances simultaneously; no stealing takes place. Failing to steal an instance when it is necessary to strictly limit the number of instances would undermine stealing’s usefulness as a tool for resource management.
That being said, I do understand that you want the start of one sound to trigger the fadeout of a different existing sound, and that you want a feature that allows you to achieve that behavior more conveniently to be part of playlist instruments and event macros. I’ll add that suggestion to our feature/improvement tracker. We won’t change how stealing works for the reasons I’ve already said, but we might be able to add (for example) a different feature that works in parallel to stealing and does what you want.