These are technically achievable with an iPad or Pixelbook, but require effort and discipline on the part of the user and on some level directly contradict the design and intended use of both products.
I think there are still significant savings to be had in terms of power usage/battery life. Lower power hardware puts a hard cap on how much any one application can consume and incentivizes efficiency on the part of the developer. The device's screen panel can also be selected for efficiency over density, brightness, color gamut, etc since those aren't priorities.