I strongly believe that SYD adds a measurable amount of prestige to the BA brand. For one, it is part of a small group of airlines that serves all six inhabited continents. Additionally, BA takes pride in its heritage (at least outwardly) and for quite a while, they were the world's favourite airline in terms of intl pax served and they had routes to all corners of the world.

I don't think we can deny a prestige aspect.

Also, many airlines run/ran routes for the prestige aspect.
Notable examples:
- SQ 345 EWR/LAX service
- former TG 345 JFK/LAX service
- former MH KUL-CPT-EZE service
- Conviasa CCS-DAM service
- TAAG LAD-EZE service

And many others that I am certainly forgetting.