As was pointed out last night at the Forest Hill society AGM, the one issue that might arise is if it costs more to change bus at Elephant. This would only happen if the journey took more than 1 hour. Looking at the bus timetable it is unlikely that this journey should take more than 1 hour, but in peak times TfL journey planner estimates just over 50 minutes journey time from Honor Oak. If there were serious traffic disruption in New Cross or diversions then it isn’t impossible for the journey time to be more than one hour - but it would be rare.
On the 171 that might not be too bad as you would be able to get off earlier than E&C and change to other buses into Central London (i.e. 176) but the 172 doesn’t really follow the route of many other buses into Central London.
Extending the 172 to Bell Green would probably make this problem more likely as it may well take 1 hour to travel from Bell Green to Elephant and Castle during a heavy rush hour.