1. Who else can Apple put as default search engine? -- Bing? But Microsoft is Apples direct competitor on laptop and desktop market. Who knows what Bing would show users? And other competitors in search market are extinct. Yahoo, AOL, etc. Duck is using BING's api.

There are local market search engines (like Yandex in russia), but they already are default search engines on mobile in that markets(at least on lot of android devices, don't know about iOS).

At most Apple would offer user to chose search engine on first set-up, and guess what users would chose?

So we slowly are moving to second point.

I don't thing Google needs to pay for this, they just are doing this by inertia. Most competitors are extinct. And people know by word of mouth, that google "is the best search engine".

About Firefox it might be tricky. People who decided to try firefox can be more interested in trying another search engine as well, so here their payment for default SE makes some sense.