I'm not sure Jolla can find a possibility to resize the partitions which works 100% and is safe. But that's what needed to deploy such change as normal update since people will get very mad if it soft- or even hard-bricks their phone.
I've understood the whole rootfs size argument from Jollas side in a way that they will change the partition size in future images. So it will change in images for new phones, maybe they also provide a new image for the Xperia with a bigger rootfs partition (but you will have to reflash your phone). Maybe they will release a resizing script for all of those who would like to take the risk. But I deem it's to risky to include it in a normal update for everybody.