At the end of the day, the Japanese market is huge and people want access to it. Same thing goes for the US.
If the private market doesn’t want bonds, the central bank can purchase them. That’s not inflationary. What is inflationary is how the government then spends that money, but that’s true for any government spending, regardless of how it was financed. Either way, the debt ratios is literally meaningless.
So, a country is not a business but it is comparable to fish?
Are you seriously claiming that there is no precedent?
Central bank buying bonds and increasing money supply absolutely is inflationary. That is precisely how FOMOs work, with the end goal being increasing or decreasing money supply depending on inflation and labour market. So if you already have stubborn inflation and you have a fiscal crisis then unmooring inflation expectations by lowering rates is exactly what you don't want to do (risk becoming a banana republic that inflates away it's debt). I don't think this will happen in the near future but it is absolutely a risk and you'd be foolish as a central banker not to consider it.
Go high enough, interest payments consume the entire federal budget. There is no way out except revenue growth (infeasible without breakthrough productivity improvements), taxation, and printing money (equivalent to taxation). Before that point, other bad things happen such as creditors losing faith in the government, making debt more expensive and destabilizing the dollar's position as global reserve currency.
Over the last few decades, debt has continued to rise as a percentage of the federal budget, and appears that trend will continue without drastic action.