From looking back when this was previously discussed internally, it appears that the current structure (i.e. not being able to amend the ID once it has been set) was configured for Performance reasons (the way that it filters subgroups using LIKE statements). This has not changed since it's initial interation - simply because it would be quite an amount of effort to rengineer.
However, after the challenges that you and other customers have had in recent months with the changes to your org structure (especially these days, with more reliance on Organisation Components when using Service Subscriptions), I have reopened discussion of this thread with our product owners and management to see if this can be revisited.
I will keep you informed of any updates