"Cancel tx" is a powerful tool and, if I am not mistaken, Electrum is the first wallet to implement it... But does a regular user really need it?!
Electrum is the first wallet to implement such a button, but many wallets already support RBF transactions, which is all that this button does.
All I can think of is how it's going to be massively abused by scammers.
Double spending required a little bit of knowledge, but now it became accessible for everyone. All it takes is few clicks.
Any scammer with access to a search engine and 10 minutes could figure out how to make a RBF transaction back to themselves. This button doesn't really make any significant difference on that front.
Replacing a transaction by hitting the "cancel" button is only possible with transactions which are opted in to RBF and have zero confirmations. Such opted in transactions have never been safe to accept with zero confirmations.