The command line options were almost identical for an easy switch. ssh even respected the .rhosts file! Last time I checked, that functionality was still in place.

Both the rlogin-family of commands and the telnet/ftp-family were in use across the Internet, certainly in cases where Kerberos was used. I would think telnet was more common, certainly so outside the UNIX sphere of influence, but things like Kermit also existed.

They all got SSL-encapsulated versions in time, but Kerberos solved authentication for free, and for the simpler use cases ssh had already taken over by then. And in the longer run, simple almost always wins!