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Users are pretty much always able to choose their search engine. I don't understand what you mean by "give users the choice of default search engine". What would that look like? A global vote on the defaults for various platforms?

If we open that floodgate, I feel like we have to talk about the browser situation too. How are innovative browser engines expected to compete in a landscape where their defaults are not respected or possible?

on iOS the most innovative browsers are just shells over safari. Every browser is safari

At least the other OS's allow you to run your own rendering engine

The completion is a click away. On Android it's settings in the browser default search, similarly for Firefox and Chrome on desktop.