Considering testnet coins are supposed to have ZERO value, a 51% attack is not only meaningless but also pointless.
TbtcThe tBTC coins are only needed to test code, once the testing is done no one should care about them and in the past
the coins were
usually returned to their source for others to use.
The only folks who would get hurt are the assholes that 'mine' it for-profit by getting
tbtctBTC & then selling/exchanging it for other coins and the idjits that buy them. Seeing their 'holdings' disrupted certainly would not break my heart
