For ephemeral DIDs to be useful they need to be recognized by the authorities that issue "credentials" based on them. For example, if a website/app requires proof-of-age (you know, like the UK now) you could use a DID for whom a credential showing something like "this is a person who is over 18 years old" was issued by the Birth Registry Authority (or whatever they call you in your country), and the website/app could then check the signature of the credential and be sure it was signed by the right authority. As the owner of a "main" DID, you could request many DIDs (and issue "credentials" based on those), presumably one for each website/app, to evade tracking.

If there was another mechanism to prove age, for example, if everyone had their Date Of Birth on a blockchain or something like that, it could be possible to not rely on a single Authority, but to my knowledge that wouldn't be acceptable in any country of the world... only the government is recognized as an "issuer" of Birth Certificates and names, and I think that's how it has to be... that makes it possible for the government to find out which apps you're using, unfortunately. But there may be ways around that... I believe the whole Verifiable Credentials Working Group uses Verifiable Presentations for this, see https://hub.ebsi.eu/vc-framework/ebsi-w3c-vc-vp