It has nothing to do with it. It's just tragedy of the commons.
Poor people outnumber rich people, so it's only a question of time until they start stealing, because there is nothing stopping them.It's worst than a bank security, since at least the bank has a vault door, stopping the mob. Bitcoin doesn't.
They just download modified software, and overwrite the consensus that easily.
Do you really think that if Bitcoin could be stolen en-masse that it wouldn't have happened already? Do you think that thieves aren't already adequately motivated?
The average thieves have low resources. This kind of theft requires a large organized coordinated attack, which hasn't came yet, but it could in the future. Economies getting worse, while people are getting savvy enough to learn online mischiefs. Expect this attack to happen in the next 5-10 years if the community doesnt do anything against it.
How is anyone sign a transaction without the private key for that particular account?
Good question, that many guys on the forum cant comprehend.
By changing the entire protocol to allow the coins from your address to be moved with the private key of the attacker, and not yours. It is possible. And the majority of nodes can do it.
Or a less complicated method is to just change the bitcoin inflation, from 12.5 BTC to 12500 BTC/ 10 minutes, and hardcode that to a way that the BTC goes to the attacker address instead of a miners.
If changing the protocol were a trivial thing it would already have been done - why do you think there's such a contentious a blocksize debate? Also, your assertion about nodes being able to sign transactions out of accounts for which they don't possess the private keys is wrong - to the point of being nonsense.