It’s the same problem with every other monopoly. Everyone wants it, both shareholders and customers. It’s objectively more efficient to standardize on the same equipment or software, train workers on it. It’s better for workers too since their skills are transferrable. It’s only bad when the negatives show up, such as price gouging by the shareholders, or them cutting corners in quality or security. But my point is that not going with a single vendor isn’t free on all sides of the equation, it requires work, which is why on average we tend to prefer monopolies even as consumers.
To put it bluntly, I really don’t want to have to think about grocers profit margins and prices after having worked 9 hours. I just want to get fucking eggs and bread from the store nearby. I don’t want to drive or bus ride to another one. It won’t happen. And that’s why it doesn’t. The assumptions about the individual (constantly shopping around for the best price) in the mainstream microeconomic theory are just wrong. This translates into small businesses (not only) shopping for their dealer sales software system.