When the blockchain undergoes a hard fork you are basically doubling the number of crypto-currency units that exist and all holders will own the original coins and the new coins (assuming they have their original coins in a personal wallet where they know the private keys rather than on some online exchange that may or may not support the new blockchain).
This duplicating of the blockchain is similar to a company doing a 2:1 stock split with one major difference - when a stock splits 2:1 the price is automatically halved and all current owners automatically receive twice the number of shares that had before because the original shares and the split shares represent exactly the same thing - just like if you had a pizza and cut it in half - you still have 1 pizza but in 2 pieces that are half the size.
When the blockchain forks the new blockchain is NOT the same as the original one in some very major respects:
So, all this goes to show that a hard-fork of the blockchain is not like a stock split and more like a spinoff of some fraction of the company that has a value of some “fraction” of the original, depending on exactly what has been “spun-off”.
In the case of the hard-fork in August 2017, subsequent trading of BTC/BCC showed that the “value” of the bitcoin cash blockchain ended up being about 10% of the original BTC one - so it was equivalent to a spinoff of 10% of a company using the equity example.
Note, also that once something spins off then it trades independently of the original and the 2 things basically have nothing to do with each other - if you owned company A and it spun-off company B, you would own both A and B in your brokerage account and they would trade independently.
So there is nothing inherently “dangerous” in hard forking the blockchain but if this causes processing power to stop processing the original blockchain and shifting to the new one, or some functional enhancement is introduced that makes the old blockchain obsolete or much less desirable than the new one, then there could be a switch to a new blockchain that makes the original BTC obsolete and therefore valueless; once a blockchain has no processing power being applied to it then it will simply fade away because there will be no-one mining new coins or confirming transactions.
The main “danger” is in user confusion if too many “bitcoin” blockchains exist, operational issues due to the complexity of all participants having to implement and accept multiple instances of each blockchain, and increased risk that some functional enhancement that has unintended consequences or breaks a particular blockhain is introduced by mistake.
