What word? In normal discourse everyone knows you're talking about long term policies so unsustainable equals impossible.
Are you sure people commonly used the term impossible for things like m4a or cancellation of student loans, and do so literally?
I think the idea is the cost outweighs the benefit in non emergency situations.
Anecdote time: I have been told numerous times, by US physicians and nurses, that m4a is impossible. That it literally is not achievable in the US. These weren't even hardcore republicans. Centrists for the most part.
And I have been told the opposite. Even professionals are human. They have political views, just like the rest of us, and they don’t always agree.
Hyperbole is a feature of political discourse in the US, let's not get semantic.