The biggest existential risk to the web is that it’s not good enough. It’s being threatened by sharp innovation in other spaces - may the Vision Pro never come down in price or improve in UX.
The biggest existential risk to the web is Google themselves: it's the world's biggest advertising and user tracking company having outsized influence on where the web is headed, and moulding the web for its own profit.
And while they are "moulding the web for their own profit", at least they are interested in keeping the web alive, and I don't think anyone interested in the health of the web will like the alternatives, which are actually winning right now (closed app stores delivering native apps with spyware and unblockable ads).
By doing this, the DOJ may invalidate one business model that keeps certain FOSS projects alive — commoditized complements to proprietary products and services. You can hate Google, and still see why this will be a huge problem, especially for projects as complex as browsers are.