It's not clear how mandatory state responsibility actually is. For example, the OST states what conditions cause a state party to be responsible, but does it say every activity in space requires a responsible state party? What about activities by non-parties to the treaty, like Suriname or Tanzania?

It's also not clear how long liability actually lasts. For example, the United States might be liable for the first mission to Mars, but who is liable for their children and grandchildren? What about a city of a million people on Mars, with people of diverse backgrounds, and a mix of native and non-native population?

FWIW, I am a lawyer and studied OST in law school. This is a very grey area of international law, with a lot of blank space, and the presumption of state responsibility for all activities in space is being gradually eroded as our understanding of the space sector shifts from the Cold War context of the OST (where there were functionally only two spacefaring powers, and all missions were temporary, without any goal of permanent settlement) to the modern ambitions of people like Musk and Bezos to actually settle other planets on a permanent basis.