The issue is not discretion. For that normal rules and sensibility can apply.
The question is simply: If you taxi out with intention to take off then have to come back should it not be a sector?
If you chose to extend you need to know what to extend upon. Depending on time of day, location, previous duty and what actually took place on the day I may not extend or give full extension. But this is not the issue.
The other aspect is the pay issue: You could extend the duty but the flying time could be less if they change you to another destination and recrew the original flight. This often happens as time has been lost so original destination may be unrealistic.
Now you've done an extra sector that has not been counted, possibly extended the duty and wind up getting paid less for your efforts.
That's not right. I'm back to the old argument that fatigue and pay should be related to duty not flight time. Sectors should simply limit the duty time.
If you work you get tired. If you work you should be paid.