1. The Japanese bubble already burst in the late 80s and 90s. They are in stagnation ever since, they are not getting worse or better. Meanwhile, Japan's debt is internal, held by japanese citizens, japanese investment firms etc..
2.The European countries debt is also not really that severe or dangerous since it is backed by the ECB and the various mechanisms established during the 2010s crisis and then the pandemic. Even the greek debt is not that bad.
The percentage of greek debt to gdp is that high because the gdp has shrank around 35% in the previous 12 years so it does not reflect the debt in absolute numbers, greece btw has a positive primary surplus since around 2012 i think. Also, the current government has changed many of the previous administrations' policies (especially the dreadful 4 years of the previous radical leftist government which caused the disaster of 2015) so the economy is bound to bounce back sooner rather than later (greece is now one of the fastest recovering countries in the EU after the pandemic). They also replace previous debt and with new bonds at much lower rates (the 10 year bonds where above 6% not even 5 years ago, now they are around 1% if i remember correctly).
3. If you really want to look where the next big debt bubble that will destroy the world economy is, look at china, not at the central governmental level but the local governments. It is there where they bury their debt so that it doesn't show as central government debt, and it has been in the trillions and growing uncontrollably for nearly a decade.
It's funny how you are talking about "the debt is fine if it is held internally" but then go write about the "debt bubble that will destroy the world" in China when their debt is also held internally - primarily by the government and SOEs, not even the public. Don't you see a contradiction there? The external debt of China is small and it is a net creditor, just like Japan.
You are also incorrect about the local government debt - it is counted separately from the central government debt and the total figure is a sum of those two. The lack of transparency in LGFVs is true, however, the estimates have been in the $4-5 trillion range, which even when added to the government debt figure would lead to around 65% of GDP which is not that big.
Inb4 "300%" comes up without mentioning that it stands for the total non-financial debt (non-financial corporate debt + household debt + national debt) and that figure was ~270% in 2020. For instance, the US non-financial debt was 235.5% (non-financial corporate + household debts) + 129% (national debt) = 364.5% in 2020. In fact, many European countries are 300%+ on that metric (Italy 180% + 155.3% = 335.3%, France 296.5% + 115.7% = 412.2%, etc. with likes of Germany 176.9% + 69.7% = 246.6% being the exceptions).
