Well, here is a rudimentary test:
This is from the book Triumph of the Optimists 1900-2000
The average return is 0.71%. So it appears that bonds have a positive return right? Except this is all the data that is avaliable (at least to those researchers). There are a huge amount of countries where there is no data. But the sucessful countries (like Switzerland, the US, Canada) there is plenty of data. In fact, I dont think there is a stable bond market (as measured in a 100 year period) that is not in that sample, and if there is, its a small number (maybe one or two).
But there are a LOT of failed bond markets that are not there. Where is Uganda? Where is Argentina? Greece? The more I look at data, the more I find that success in bond markets is very rare, its the exception not the rule. So, if I had 100 year data of all countries, its very unlikely the return would be positive. Most of the countries not in there (unstable countries), had several currencies, several inflation crises, several debt default crises. Effectively, they repeated the experience of Italy/Germany/Japan/France (which have negative returns), they had one or several huge drawdowns that bond markets need many decades/centuries to recover from.
So, it looks to me that its quite unlikely that if had 100y global data, the return would be positive. Its not like sucessful bond markets can remains hidden for long. Sucessful countries become known and turn into 'havens' for capital (even if the country is small), its failure that gets hidden/dissapears