Scp was, according to its manual page, based on rcp (part of the rsh/rlogin set of programs): “scp is based on the rcp program”.
Sure, but scp uses the same port as ssh so the underlying protocol has to deal with both terminal and file copying commands.
Well, yes, exactly: rsh, rlogin and rcp were often considered together, as they both used the same access control - /etc/hosts.equiv. So it's not that surprising that ssh, slogin and scp came together as a replacement set.
And don't forget sftp!
Sftp was not part of the original SSH program, IIRC.
I agree. I first started using SSH in 1996 or so, building it from source on a Linux Slackware system (which didn't even have ELF binaries!)
sftp didn't appear for a few years later.
I remember porting my system from a.out to elf binaries. Manually. Learned a lot though.