> It's funny because most of your argument assumes setting some kind of threshold would be a problem, as if it was not exactly how fiscal rules worked already.
But it is the crux of the problem.
When establishing things like "country of residence" (or, here in the US, to which state(s) you owe taxes), the second order effects aren't the same. It's the difference between "you owe a few extra percent of your income for one year" versus "you lose your home permanently".
As public policy, it is important not to expropriate someone's home simply because they had to be away for extenuating life circumstances like caring for an ailing relative for a few months. Because otherwise, you will disincentivize that behavior (and create a greater burden for the state).
It's easy for you to come back with "Oh, well the competent bureaucrats in my government will simply write an exception for those who leave for a few months of eldercare because that is clearly a legitimate reason to leave your house vacant for a few months." But that is just a bandaid on an artery which creates an explosion of exceptions and exceptions to the exceptions.
What happens if I need to care for my grandmother, and then something else comes up? Do I get to have two exemptions to expropriation? Can they be consecutive? What sort of documentation do I need? What if the exemption allows n days, but I need n+3? What if I had planned an m-1 days vacation (for the m vacation days exception) right before I need n days to care for my grandmother?
Very quickly you will have 10,000 pages of rules that nobody understands. And you will get perverse outcomes when someone hits an edge case that wasn't quite accounted for.
Or maybe you will say "put the case in front of a judge and let them exercise discretion". But now we have the problem that it is no longer really rule of law, and people with connections will always get exceptions while people without don't. So that increases the class divide because people with connections are usually rich, and middle-class people usually don't have connections.