I think you are overestimating how decentralized the web is. Tailscale (could) collect a different kind of data than e.g. Google, and I believe it's far less harmful for the average user. If you are a journalist or persecuted by a government then sure, Tailscale is bad news.

But if you think Tailscale is bad for mobile devices because they might be able to tell that you're near another Tailscale user, that is absurd. If you have a need to be physically untrackable, the only solution is to leave your tracking device at home.