Leaking bits of individuality is one issue.

The other is that once a tech choice becomes too niche, it stops being supported:

- Technically anyone can run their own email server but from what I hear if you do, some providers will treat you as spam.

- Niche features get removed from products.

- Some niche usecases depend on legal support. Running programs on a device you own without going through a gatekeeper ("sideloading") may be required in some jurisdictions (EU, any others?) but there's nothing stopping the almost-monopolies from making it impossible elsewhere.