The risk of a vastly superior design would be unlikely to collapse bitcoin, because stakeholders still likely consider their bitcoins to have value -- and that's the fundamental underpinnings of any currency including bitcoin, shared idea of value.
I'm not sure I agree with this, unless you mean that the stakeholders will likely still consider their bitcoins to have value
temporarily. However, if it is well known, even amongst stakeholders, that a superior currency exists they will gradually migrate towards it (even if they tell each other otherwise).
There are many things vastly superior to gold, but it continues to go up in value. When people are looking for a safe place to store their wealth they are not analyzing technical merits. They instead look at the historical value, which is exactly why we are having this conversation right now. Bitcoin is new and people are concerned about its future value since it has no history.