Two years ago I was in a conference connected with crypto and it was said that there had been no such a powerful computer that could destroy Bitcoin's blockchain yet, so I think that in terms of it we are in safety now. What comes to polynetwork, it is just a defi-project, so hacker found an vulnerability in its smart-contract and stole money. But there is no Bitcoin's smart-contract and if someone changes something in Bitcoin's code it just will not be accepted by other computers.