all coins held in SegWit addresses will be donated to miners, as they are free-to-spend coins in the legacy (pre-SegWit) blockchain, and only the coins held in legacy addresses will stay intact and belonging to their owners (private key holders).
Whaaaattt???
Good thing my stash has never touched a segwit addy then eh? lol
I'm safe..
There is zero actual truth behind this. We're talking about a 51% attack, in which it would be retarded to think the price of BTC would come out of it unscathed. However, the chances of a 51% attack happening in BTC are next to nothing.
Even if it did happen, assuredly the bitcoin community would reorganize and go with the chain where the attack didn't happen. So "anyone can spend" would only be in effect for as long as people were willing to buy coins on the wrong chain. It would be chaotic for a few weeks, and yes, the price of BTC would plummet, along with all other coins. It could take years for confidence to rebuild afterward. However, I'm willing to bet anybody that this will NOT happen, EVER.