We don't even need to assume a perfect internet where all hosts are always connected, we just need to assume an internet where all hosts are who they say they a

We don't even need to assume a perfect internet where all hosts are always connected, we just need to assume an internet where all hosts are who they say they are, and there is no way to forge your source address. In that case, yes, you wouldn't need a three-way handshake. The client would send a SYN, the server would send a SYN-ACK plus any data it wants (e.g. an SMTP banner), and the client would send any data it wants as well with the next segment (e.g. an SMTP request). If the server doesn't receive anything, it could re-send the SYN-ACK+data, just like any other packet. The reason this isn't done in reality is mainly that using a server's resources to send a lot of data blindly, in response to a very small request, is wasteful (if the reverse path is not accessible, the server is using all those resources retrying for nothing). It is also dangerous, if the source IP of the SYN is forged, then the server ends up sending lots of data to a victim's IP. This often happens with DNS, for example.