10W,
Sorry, I see what you're saying now. However, the point still stands... an alternative to a system would generally perform the same function as that system (therefore being used in lieu of said system). ACAS and ADS-B are two separate systems, designed to carry out to different functions, and therefore cannot be used as alternatives. The CAA (Cargo Airlines) are looking at ADS-B for conflict detection, but it will not be such a 'last-minute resort' system as TCAS/ACAS (i.e. ranges of less than 7NM).