The debt to gdp ratio in Japan has been well over 100% for decades. In fact it is over 200%. What matters is the ability to service the debt. Those buying U.S. government securities are not as alarmed as you. Do you have reason to believe that they are lacking in knowledge/understanding of the situation and are wrong? They could be making huge mistakes with their willingness to lend but I don’t know enough about this stuff to decide if that is the case.
https://en.wikipedia.org/wiki/National_debt_of_Japan