If you receive a transaction on your node, and it is confirmed, you will regard it as a good one. The coin being spent may not exist on the other chain, or there may be another transaction on the other chain spending it differently. So you will lose coins, and can easily be scammed.
That's not what I'd call "losing coins because you run a node".
Great. I'll tell pepole who lose their coins that you have chosen a different name for it, so it must be OK.
That's evident that if the chains have identical signature schemes, and you didn't do anything to split them, of course a valid transaction on chain A will also be a valid transaction on chain B, the famous "replay attack". The thing to do, is that ideally, the one forking off should modify something in the signature (hard fork !) so that this is automatically solved (a good signature on one scheme will not be a good signature on the other) ; or you should mix in yourself newly mined dust on one of the two prongs. Otherwise, it is normal that any signature on one chain will be copied over to the other chain.
Yep, so you will make two coins. And if you were to do that for every minor improvement, we would have more than 20 different bitcoin blockchains already, and most people, including me, would see it as a failed experiment.
If it was simple to change the hard economic consensus parameters, like block size, inflation rate, time between blocks, POW algorithm etc, it would have happened several times already. It doesn't, because people want bitcoin to be a secure store of value.
It doesn't, simply because of the mechanism of immutability, which, however, can break down if centralization occurs and there's a collusion of more than 50% of the consensus (= hash) power over a change.
> 50% of hashpower can only restrict activity by refusing to mine certain transactions. They can not change the consensus. If they produce invalid blocks, the hashpower is worthless. Nodes will just throw their blocks away.
More than just restrict activity. ANY soft fork is automatically imposed by a >50% hash rate collusion over the soft fork. If tomorrow, >50% of miner nodes decide to impose segwit, then segwit will be. Other miners have no option. They will get orphaned all the time according to their own rules. Because all nodes will accept segwit blocks like "legacy nodes", including the non-segwit miners.
Of course, because segwit is a soft fork. Hard forks don't work that way. The blocks produced on the other side won't get orphaned. Both chains will continue as different coins.
A hard fork is a whole different affair: you make two coins, and users happen to possess both of them. Up to them to use both of them, by running their respective wallets (and eventually, their respective nodes).
Yep, and if someone want to do that with bitcoin they are welcome. Just don't call the new coin "bitcoin", because that will be confusing to everyone, and don't expect anyone to use their new coin.