You’re right that it’s “a hundred different pieces each doing it right” but you gloss over the disadvantage of decentralization: getting everybody on board with the project. With a centralized product (such as at Apple) the CEO can say “I want to increase battery life on existing hardware by 20% before next release or you’re fired” and people will work 80 hours a week to get that done. With open source? You’ve got ten thousand different projects, each with their own leadership and their own priorities. You have no leverage at all to get them all to work on power efficiency, so naturally it takes a back seat to them working on their favourite features.

Because it's not actually a disadvantage, because if it's actually open source no one can stop anyone else from doing it. If you or your company wants to work 80 hour weeks to improve power efficiency on Linux, you can submit patches to all the different projects where nobody else is doing the work.

And that actually happens in real life. Most of the projects care to begin with because they use their own stuff and don't want to ruin their own battery life or have a competitive disadvantage over the alternative. Then other third parties that find business value in having it work pay someone to clean up the odd stragglers when one of them didn't do it or have the resources to do it themselves.

The main problem is when some vendor both doesn't do it and is hostile to anyone else doing it.