I'm not sure where the 98% has come from. It may be that 98% of Mode S equipped aircraft are capable, but whether the ID entered bares any relation to the ICAO Callsign or aircraft registration is another matter. A number of operators are not entering correctly, either because of cockpit procedure problems (or lack of!) or avionics faults.
Until just about everyone is carrying a Mode S ELS transponder and entering the callsign correctly, the benefits won't be fully realised.
RS