Yes lots must have lost money in the way you described but there are solutions to avoid it (watchtowers). In other words: This time the cheater is discouraged to cheat. It's a huge step to move from a completely trusted debt-based monetary system to a trustless one. Agreed?
watchtowers.. ok now you are creating bank managers.
so now people have to trust their channel partner to not cheat. and then trust a bank manager to watch that the partner has not cheated. and hope the watchtower server does not glitch out and go offline one day, thus not able to watch and activate punishments.
oh and watchtowers can only intercept and change ownership if you trust them to have the keys to do these interceptions on your behalf..
so where is this trustless system you mentioned?
in short
my keys my coin. not my keys not my coin.
multisig requiring third party signing is not trustless independence