Who in a decentralised system can or should take responsibility?

The customers. :) Ear me out:

The main reason why there is so much problems with deliveries (way more with UPS, DHL and the likes than Uber Eats deliverers, in my own experience) is because we're not their customers, in their heads. They're paid by the merchants (UPS/DHL/etc are paid by your shop keeper), or they're paid by the platform (rider is paid Uber Eats/Deliveroo/etc), but the end customer is just part of the constraints, for them, especially since the customer doesn't even choose who will deliver their package (you don't like UPS? Too bad!). Give the customer that choice, and make them pay directly for the delivery to the deliverer, and I guarantee all those problems will go away. This is why I said we need a decentralized reputation system : so that the customer can see the reputation of local delivery service before selecting them.

When the problem is with the shop, well, this is already sort of dealt with. We already have reviews systems and we already select our shops, so it does happen that shops behave poorly, but not for long. Although, users have to be educated about verifying reviews, and developers have to implement countermeasures and stay on top of the review cheating game.

And to avoid problems with the platform, we have the interoperability of standards like ActivityPub : there is one global network (like the fediverse, or the web), and multiple programs are implemented to use it. They have a incentive to work well because there is competition, something that centralized platforms eliminate altogether.