SlAvA UkrAnI!
What part of martial law do you not understand?
A country being invaded can be and will be overthrown if possible. In fact, it's been done many times in European imperialist history.
That's why the clause exists, even before democracy was normalized in Europe. Just find someone else in line for procession and install a puppet prince.
It's even been abused. Some speculate that Trump would trigger martial law to stay in office - or even Netenyahu himself clinging to power.
In the end you don't want a captured government. That's also historically been really bad.