Too late now, ICAO has well and truly screwed the pooch.
A bit unfair on ICAO, who originally nominated three (3) systems as potential candidates. 1090ES was soon eliminated in early trade-off studies.
It was lobbying by transponder manufacturers, who did not have control of patents for modern broadband data links,(UAT/CDMA or VDL/TDMA) who (along with major, mostly US, airlines, who swallowed the argument that 1090ES would be cheap and easy) were successful in promoting the present financial and operational mess --- channel saturation is already a major issue in the US, as clearly predicted by Mitre Corp. research, so many years ago.
We all know about the cost issues --- at least the FAA cost estimates were realistic, unlike CASA.
If UAT (or VDL-4) had been adopted, we would not be in the present situation.
Tootle pip!!