Why Zoom doesn’t have product/market fit

He might have a high fraction of people who understand that Zoom has no lock on the market; these folks will use whatever it is they are invited to. So long as they will tolerate Zoom, that is good enough. The real market is the person who buys the license. For instance students and teachers might not think about the administrator who bought a license for the whole school.

Would be fun to compare against competitors in depth.

I have a hunch that "chat" programs have been going sideways for a long time because market share is based on lock in. The platform rots because it is not competitive. At least until a new fad comes along.