I have also read and heard from different financial articles and podcasts that there is no known upper bound to amount of sovereign debt a country can have before keeling over - it is more important that creditors and investors have faith that the government can pay it off, or that the country is good for the money.

However, I tend to agree with your comment. There may be no upper bound to sovereign debt, but economic good times can't last forever, what goes up must come down. And should creditors ever need to collect that debt there must either be high inflation or high taxes, or not pay back loans and risk sanctions.