The incentive to keep iOS safari competitive with Android browsers isn't as strong, as it would be if iOS Safari would have to compete with browsers on the same platform.

Significantly, the lack of features on iOS Safari, which might be present on Android, makes using these features equally infeasible just as effectively. Nobody wants a mobile webapp that only works on Android.

Lastly, the health of the web should not be governed by the rise and fall of mobile operating systems. Android or iOS may rise or fall, and with it, competitive pressure for the predominant browser vendor would change.