Airbnb is not hotel management, just a way for someone like me or you to make some extra bucks. Fiverr is a marketplace for professionals to share their services online. You have to work here, it’s mostly active income, for Airbnb you don’t need any skill, you don’t have to work on anything, just offer the free space you have for $$.
Not sure why you are comparing with Airbnb. Anyway, it’s my opinion and I for one I am sure Fiverr will never implement this as it would cause a lot of friction and wasted time on their side. The return will be minimal for them, but adding this would drive buyers away. They need to bring more buyers here, not to drive them away. An automated process would help but as @humanissocial said, the best thing they can do is to not have us sellers deal with penalties. Then again, if sellers see they don’t get penalized if they choose this option, that would make them abuse the system and say all orders they want to cancel are placed by mistake. I believe that’s why they removed the lack of penalty from mutual cancellations in the first place, to prevent abuse.
So, I would say don’t expect any decline/accept feature anytime soon because it requires a lot of work and planning with pretty much negative returns. Again, my opinion, but I am 100% sure Fiverr will never do this as it doesn’t make any business sense.