If it is sustainable depends on the quality of investment returns. But that phrase abstracts a lot - the quality of things/services/business produces for that investment need to lead to new or improved productivity in economic activity.
It also may take a longer time frame for the improved productivity to manifest than it does for the economic metrics to reflect it. Meaning that one thing to watch out for is that the capital abundance may recede before the investments are able to return the productivity - and that would be tragic because there are an abundance of neglected investment areas which we as a society and an economy can realize benefits. To the extent that our capital investment allocation paths don't match societal needs is the real challenge with this bubble.
It also may take a longer time frame for the improved productivity to manifest than it does for the economic metrics to reflect it. Meaning that one thing to watch out for is that the capital abundance may recede before the investments are able to return the productivity - and that would be tragic because there are an abundance of neglected investment areas which we as a society and an economy can realize benefits. To the extent that our capital investment allocation paths don't match societal needs is the real challenge with this bubble.