It was the only one at the time. Since then a few alternatives have been proposed but none that rival PoW, at least in terms of security. > Blockchains are not

It was the only one at the time. Since then a few alternatives have been proposed but none that rival PoW, at least in terms of security. > Blockchains are not a complicated concept. They are merely a merkle tree combined with a consensus algorithm. That's it. I wouldn't say so, the block chain is a chain, not a tree. Actually, the use of merkle trees to represent transaction data is an implementation detail. Bitcoin would have worked fine without them (though "lightweight" clients would have suffered a bit). I second that it's a simple concept though.