I do not need to elaborate on why a diverse browser ecosystem is important, there are enough threads on that already.
Mozilla had this jewel, exactly what it needed to succeed and make the web a better place. He was attacked for his personal beliefs regarding gay marriage, which went against the current popular liberal attitudes, who attack anything they perceive as against them with a fury on the scale of lynch mobs. The web, for better or for worse, gives huge leverage to whoever is willing to make the most noise and be the most unruly.
So Eich was attacked, Firefox market share plummeted, the new CEOs take home four times what Eich was taking home yet add "Pocket" and other misfeatures while funding for Rust and MDN is being cut. Instead of a 95% monoculture of a Microsoft product we now have a 95% monoculture of a Google product. But hey, the guy who doesn't support gay marriage is out of the picture, right? That's a win, right?