These a/c get turned around in less than 90 mins. Disconnecting the power for 30 mins while you change the batteries would kill that.
1) If the battery is isolated the A/C does not need to be powered down at all.
2)The battery would only need changing if used, capacity indicators are not rocket science.
Then there is the problem of transporting hundreds of batteries all over the globe.
Alternatively you have charging workshops on every line station.
I can't see the airlines wanting to carry the cost of any of that.
Cheaper for Boeing to pay for that than having them as expensive ornaments twiddling their wheels in a hangar and it would take some of the pressure off of providing a fix. That way a proper engineered solution could be devised and tested.