As parent stated, Google can simply neglect to make their apps work well in Safari. They’ve done so in the past; GMail on Firefox was nigh-unusable due to a raft of dumb bugs for a long time, and it still uses substantially more CPU and memory than literally any other tab I have open on my browser. Right now they have to make an effort to support WebKit (Safari); as soon as they can push Blink (Chrome) to users, that motivation will go away.
In a discussion about encouraging browser diversity, are you seriously arguing that it’s an unreasonable burden on web developers to ensure their websites work on more than one browser engine?