Not sure why the network would be busy when the only dangerous move would be sending a tx, particularly during a time with a full mempool. Meanwhile, this would be a very dedicated attack, and it'd be odd given how much more valuable so many other things are. Even at $1M bitcoin becomes a $20T asset. Global banking systems, nuclear launch codes, or heck, trusted trade secrets all become much more juicy targets, especially given that Bitcoin loses value dramatically in this scenario. Given that nation states need Bitcoin more than most plebs even do (whether they realize it yet or not -- to defease their debt) this nightmare situation just doesn't seem to reflect anyone's actual incentives.