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.
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.
Here's my advice. It's the same advice I gave my three teenage daughters, and it helped all three land jobs they wanted:
Don't ASK for help. Instead, OFFER your expertise and wide range of diverse skills. Don't ASK for food and shelter, SAY you're willing to be paid in food and lodging.
And so on. See what I'm doing here? Come at it from a position of strength. Clearly set up a win/win situation.
I'd re-write this post, this time sounding more in control and with emphasis on what you can deliver. Hope this helps.
Even with the benefits paired down, Amazon Prime has been a huge upgrade for me. If wouldn't be exaggerating if I compared it to going from dial-up to broadband Internet. Maybe because I read a lot and impatient to boot.