Centralized systems requiring trust are
You are presenting a false choice. The choice is not "centralized systems" OR "everything in Bitcoin". There are many options to build non-centralized ways of settling payment on a decentralized Bitcoin ilpirata79 linked to one such approach. A lack of care into the operating costs of Bitcoin nodes may result in Bitcoin itself becoming effectively centralized, which is an outcome which would make building a non-centeralized payment method on top of Bitcoin pointless or impossible.
The fear of bitcoin becoming centralized because of "heavy" nodes is imho completely unnecessary. The traffic and storage technology can be maintained by small groups, it is not like there will be "one big government node" in every country. It is more like having a node in every town.
Any system that requires a system on top of bitcoin for payments is a threat for bitcoin and it is not necessary. The basic, existing system of bitcoin allows to replace visa if it is adapted to a bigger volume.
Anything else will make Bitcoin complicated and I know I am repeating this, but one huge factor of bitcoin is, that I don't need an intermediary.
Keep Bitcoin simple, complicating systems unnecessarily has never been wise.
A node in every town is risky because it is easy to take down. Who is going to administer such nodes?
The idea is that the max block size is going to increase, but slowly. Maybe a x10 multiplication of the soft and hard block sizes could be a good start (maybe even risky).
At the same time, to be on the safe side, it is good to develop safe and cheap distributed tecniques to handle micro transactions in order to avoid the unnecessary inflation of the block chain. Like the one I posted and that invite you to read.
It is not an unnecessary complication. Other crypto currencies may arise that handle things better because are better designed and more sofisticated/complicated.
Best regards,
ilpirata79