It makes the act of pasting a replacement subflow into another instance reasonably simple to do, but it is a manual operation. Suppose, for example, that it is used in 20 systems, then it becomes onerous.
In fact the option to pack a subflow as a module is definitely the way to go, I think. If I understand correctly, the source could be on github and it can be updated there and all that has to be done is to use npm on the secondary systems to update to the new version. Therefore the update could, for example, be updated automatically using tools such as ansible, which is how I deploy flows in the first place. I had a recollection that I had seen something about this but had not been able to find it. Thanks @Hopperpop for pointing it out. I will give it a go and see how well it works.