Sorry, I meant " I don't think it would become a real problem."
However, if someone with massive hashing power could push it over the boundary, then blocks would stop, and stop forever. The next difficulty adjustment will happen after 2160 blocks are found, which will never happen.
That's under this assumption of yours:
Let say due to defect in SHA256 it is impossible to find a block if more than 160 leading 0-bits are required.
I don't see any basis for the assumption?
Why might there be a certain threshold?