That would be a discussion to have regarding mainnet inclusion, which I believe is now handled over at the eth1.0 specs repository now. That being said, step 1 for deprecation is to provide a new option. The sooner we do that, the sooner we can start the next steps of the deprecation process (such as defining an end of life date and a mechanism for slowly killing off stragglers).
Yes, it is commonly used to create deployment transactions that can be executed on all chains at the same address via a vanity signature (create signature by hand, recover address, send ETH to address, submit transaction). There are EIPs out there that provide a bettere way to achieve that, but they aren’t available yet.
Even if we didn’t want that, in order to ensure that someone with a signature from before EIP-155 can still submit that transaction to the blockchain we have to continue supporting both formats. There is a strong desire to ensure that someone who has an old hardware wallet or something doesn’t lose the ability to sign transactions.
Agreed! Unfortunately, I’m not the right person to do this. Would love to get some help fleshing out the Rationale by someone more knowledgeable than I on why SSZ!