How big of a threat is this realistically, not just in theory?
For SPV wallets like Electrum or Sparrow, the risk is very small. This is because an SPV wallet still downloads headers, so an attacker would still have to have the ability to fake a valid header which involves mining a fake block (and therefore losing the block reward), as well as have the ability to control the multiple nodes these wallets connect to. I'm also not aware of any such attacks on such wallets.For wallets which depend entirely on one company's servers or nodes though, or closed source wallets that could be doing anything at all, like Trust wallet, Coinomi, Atomic, and so on, then the risk is significant. We have seen endless numbers of users losing coins from using these wallets, and it would be near impossible to prove that such an attack was responsible for any of these losses.
