Why isn't the default email reply behavior "reply all" vs just "reply to sender" as is the current convention?
Read about email storms and you will understand why https://en.m.wikipedia.org/wiki/Email_storm
Do you want to the default to be "reply all"? If so, why? How would it improve email for you?
Because the world is cruel . I am a big fan of reply all. I would like this to be a default option
Depending on your email client, and possibly said client's configuration, it might be.
That said, accidentally replying all with sensitive or embarrassing information is probably worse than accidentally sending a reply to one person instead of everyone, so it is a safer default, even if reply all is more common.