It's already available on every apple device under low power usage. Battery life always depends on your usage and screen brightness if you use power hungry apps games etc then the battery life will be lower the same as any mobile device.
The windows laptops with desktop DDR4 ram have terrible battery lives or are big and heavy or both in many cases. No apple would make very little on a fat heavy laptop with 32gb RAM its not worth their effort despite a very small very vocal online minority most people just don't need it in a laptop. If that is what you need for a specific use case buy a windows laptop.
Also people using laptops plugged in most of the time are making a compromise that is theirs to make but makes little sense when they could get a much more powerful desktop and a tablet for when they are away from their desktop. With remote log in and better and better tablet computers a laptop that spends most of its time plugged in is waste of resources on a set of compromises that have to favour the form factor of a mobile device.