I think we first need to agree that competition in a general sense—that is including browsers and everything beyond—is a good thing. It's healthy for any market to have viable competitors; principally for the consumers, but also for the vendors (so that they do not stagnate). If we don't agree that competition is healthy, then I don't believe it's possible to agree on the second-order matters.
There are several second-order reasons why a monoculture is damaging for the browser industry. I will give a few examples:
1. A monoculture encourages web developers to test on a single platform with the assumption that all other platforms, and even industry standards, are meaningless. Many of us are web developers, and most of us are guilty of dismissing low-share platforms during testing. But as long as some viable competitors exist, we nominally target the industry standards in the hope that by doing so, the platforms we do not test on will have a modest/tolerable (maybe even good) experience.
2. A monoculture is self-reinforcing. Losing established viable competitors makes it more difficult for new competitors to enter the market. If we allow second-tier browsers to be rendered meaningless, we all but ensure a long and (eventually) painful stagnation, and create an ever-larger hurdle for a new entrant to clear to reach any significance (as more an more of the web is designed to work with a single platform). While it may sound passably agreeable to have a Chrome monoculture in 2019, do we want to still have a Chrome monoculture in 2029? For my part, I hope we see an increasing set of options in most areas of life over time, even in "browsing," whatever that ends up looking like in 10 years.
3. We don't know what future stagnation will look like. We don't know what opportunities we will have lost by making it more difficult to compete or by losing the healthy diversifying force of competition. It's a classic problem of the unseen. Projecting forward, we predict testing will be a bit simplified, but we cannot know what innovations we'll never see (or won't see as quickly) because the hurdles for experimentation were too high. Today, if Firefox introduces a new feature, even though its usage is relatively small the usage is still large enough in absolute terms that the innovation isn't entirely under the radar for most of us. If Firefox's share were 0.8% instead of 8%, far fewer of us would even notice if they added a slick new feature, leaving the feature unheralded and obscure despite its potential wide appeal.