Any railway extension is possible, just look at Bluebell the a rubbish filled cutting, the WHR or GCR bridging the gap! So forget the word impossible and use more challenging / longer to achieve.
If money is available from willing members, crayonists or grants then extensions can be built.
The key to success though heavily depends of the viability of running the service on the line built.
When the GWSR extends to Broadway, it connects to a major tourist centre and huge demand potential. Honeybourne though is not a destination, merely a connection to the "main line" with a sparse service.
The C&PRR will gain a huge market from Princes Risborough, but south towards Watlington simply extends the ride.
Missing gaps such as K&ESR to Robertsbridge will always succeed, but the Wensleydale Railway as a Garsdale to Northallaton route, although fairly easy to achieve doesn't have a base profitable railway with enough members to run it let alone fund it!
What is more likely to see is one or more heritage railways fail and close, as some seem no more than local hobbies with weak finances and if key members age / die / leave then that is it.