Supporting the most use cases is where you get the compatibility from.
Being better than your competitors is where you get browser-exclusive functionality from.
If something (e.g. ActiveX, Java, Shockwave, Flash, some new CSS that's not standardized yet, a new feature on some top 500 website, ...) works in your browser, that's a win for both goals - and if it works only/best in your browser, that's also a win.
Monocultures can be toxic in part because the disincentives to adding lockin (e.g. losing users and other negative feedback sources) get weaker/less likely as one participant becomes more dominant.
I'm old enough to remember the fun with "works only/best in IE/Netscape/...", and Microsoft's old Embrace/Extend/Extinguish policies. While I do not have any reason to believe any of the parties involved in browser development would behave pathologically, history suggests that monopolies in any space tend to eventually become pathological.
(I work for Google, not on anything browser related, all opinions above are mine personally, etc.)
You can't currently make an alternative browser in the same sense that you can make an alternative OS. An alternative browser would not use HTML, CSS, JavaScript. It would use something else in the same way that MacOS/iOS push ObjectiveC and Swift where as Android pushes Java/Kotlin and other OSes other languages and each OS as it's own UI API