I'll try and answer your last three questions in order. I'm not an expert though, only a casual observer.
- What keeps people interested in a blockchain (and currency) once all of the easy mining is done eg, for people joining later?
What keeps me and many others interested is the utopian vision of traditional banks and currencies being disrupted, and bitcoin becoming the de facto global currency of choice.
- I see that transaction costs for bitcoin keep rising, is that related to the size of the blockchain?
Raising the transaction fee gives miners incentive to verify your transaction before others. It's not directly related to the size of the blockchain.
- I thought bitcoin was supposed to be decentralised – doesn’t a group of centralised developers have the ability to change everything about the currency?
It is decentralized. There is a group of developers overseeing the Bitcoin protocol, but for any change to be made, there must be consensus.
Oh that first answer is very interesting - my thoughts at the moment is when/if is this technology going to be included by governments alongside their currencies. Eg each currency being "digitised" alongside standard paper currency. Doing some background reading, this reads as the opposite of the goal of bitcoin/blockchains.
I feel as if a more achievable goal would be not to have a blockchain as a currency or reserve currency, but rather a settlement mechanism shared between banks and individuals (who don't have direct access to ACH etc). That means the coin would be the vehicle of transmission and record. Is this a common viewpoint?
The blockchain offers many possible uses. What you describe is one. As for bitcoin, it's meant to replace fiat. It's not controlled or manipulated by any authority. What you describe sounds more like Ripple.
The blockchain is one part of the Bitcoin (with a capital B) protocol. The digital currency part (bitcoin with a small b) is what makes the blockchain run. The two are inseparable. That's my understanding.
EDIT: Ripple is an extremely revolutionary project in its own right. If anything, it's too far ahead of its time. Or maybe not.
- What keeps people interested in a blockchain (and currency) once all of the easy mining is done eg, for people joining later?
What keeps me and many others interested is the utopian vision of traditional banks and currencies being disrupted, and bitcoin becoming the de facto global currency of choice.
- I see that transaction costs for bitcoin keep rising, is that related to the size of the blockchain?
Raising the transaction fee gives miners incentive to verify your transaction before others. It's not directly related to the size of the blockchain.
- I thought bitcoin was supposed to be decentralised – doesn’t a group of centralised developers have the ability to change everything about the currency?
It is decentralized. There is a group of developers overseeing the Bitcoin protocol, but for any change to be made, there must be consensus.