Your argument is that Microsoft Office is not thriving because of institutional knowledge, then you proceed to prove that it is institutional knowledge that makes it a valuable tool.

If your arguments are: 1) Most users can pick it up easily due to familiarity. 2) Excel is a general tool usable by many disparate groups in the organization. 3) Excel is the lingua franca of manipulating data within the organization.

Then it seems to me that these would fall under the category of 'institutional knowledge', and you are in effect arguing that it is thriving because of institutional knowledge.