It seems a little silly to complain that a feature is missing from headphones when it actually exists, but the user has made an active choice to include a different feature instead. With always-on listening and easy activation from phones these days, I can't help but think that there's little reason to use the button to activate a voice assistant anyway. Not least of which is the dubious usefulness of voice assistants.
That button can be configured for that or for voice assistant activation, lots of people probably use it for the latter.