The US was a safe place with consistent returns for decades. Now the US has increased risk. It isn't a calculation of "where do we put $100B", it's a calculation of "what is the balance of managing risk and returns".

Canada and Australia are low risk, but don't have the returns of the US. Brasil, Argentina, have higher risk.

The allocations get spread across different risk profiles, and the money gets spread across different investments. Of course, this is on the assumption the majority of this money stays in Treasuries. These large funds have options beyond that. It's a mathematical calculation where every asset in the world is an option. $100B can be spread around pretty easily I'd think.

You are so far out from understanding why pension funds put money in foreign government paper that I'm not sure where to begin explaining it. But the key is simply portfolio diversification. Institutional investors have access to a very large number of options on where, how and for how long they want to park their money that it isn't some kind of forced move to put their money in US Treasuries.

It's just a way to hedge their bets. In the larger scheme of things $100B isn't all that much (it may be to you, but even a small trading desk of a mid sized bank or multinational, say Royal Dutch Shell, or such) have access to that kind of money. They are not going to take the chance that Donald Trump on Monday morning, after eating a bad burger the night before, decides to unleash the might of the US military on their territory and be left holding the bag.

One downside of running a trade deficit with the rest of the world is that they have you by the short and curlies; if they dump your paper you will have to buy it back as fast as they can dump it to avoid a crash of your currency. That can get expensive really quickly.