It would create a small financial (and convenience) pressure to use one identity. Careful design would be needed to ensure that multiple identities are encouraged and accepted.
There is enormous pressure to converge on one identity. IAM has huge network effects. On-boarding customers is an expense so businesses and governments rely heavily on existing rails like email, SSN+DOB, Facebook, SMS, etc. If you don't want to surrender SSN or your whole Facebook profile your only option is to reject the service entirely.
