The real case meat isn’t default search though, it’s the fact that they have 95%+ of the US online search market. That is stifling innovation and is part of the reason for their ad dominance as well. And they’re abusing it as well by making changes which mean they funnel you into ad results, display the content you’re after without entering the page in question thus “stealing” traffic and eyeballs and the whole amp bullshit.
Just like AT&T previously completely dominating telecom was a dampener on innovation which became super obvious once the monopoly was ended I think it will be the opening of Internet and search innovation flood gates if this monopoly is broken.