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.